Multiple directional scans of test structures on semiconductor integrated circuits

ABSTRACT

Disclosed is a method of inspecting a sample. The sample is scanned in a first direction with at least one particle beam. The sample is scanned in a second direction with at least one particle beam. The second direction is at an angle to the first direction. The number of defects per an area of the sample are found as a result of the first scan, and the position of one or more of the found defects is determined from the second scan. In a specific embodiment, the sample includes a test structure having a plurality of test elements thereon. A first portion of the test elements is exposed to the beam during the first scan to identify test elements having defects, and a second portion of the test elements is exposed during the second scan to isolate and characterize the defect.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation application of prior U.S. patentapplication, having application Ser. No. 09/648,109 filed Aug. 25, 2000,now U.S. Pat. No. 6,566,885 entitled “MULTIPLE DIRECTIONAL SCANS OF TESTSTRUCTURES ON SEMICONDUCTOR INTEGRATED CIRCUITS”, by Akella V. S. Satyaet al., which claims priority to U.S. Provisional Application No.60/197,512 filed on Apr. 18, 2000 and U.S. Provisional Application No.60/170,655 filed on Dec. 14, 1999 and U.S. Provisional Application No.60/198,035 filed Apr. 18, 2000. These patent applications areincorporated herein by reference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of inspection andanalysis of specimens and, more particularly, to defect inspection andanalysis of semiconductor integrated circuits.

2. Description of the Prior Art

In the semiconductor integrated circuit (IC) industry, there is acontinuing demand for higher circuit packing densities. This demand ofincreased packing densities has led the semiconductor industry todevelop new materials and processes to achieve sub-micron devicedimensions. Manufacturing IC's at such minute dimensions adds morecomplexity to circuits and the demand for improved methods to inspectintegrated circuits in various stages of their manufacture is everpresent.

Although inspection of such products at various stages of manufacture isvery important and can significantly improve production yield andproduct reliability, the increased complexity of IC's increases the costof such inspections, both in terms of expense and time. However, if adefect can be detected early in production, the cause of the defect canbe determined and corrected before a significant number of defectiveIC's are manufactured.

In order to overcome the problems posed by defective IC's, ICmanufacturers sometimes fabricate semiconductor defect test structures.Such defect test structures are dedicated to defect analysis. The defecttest structures are fabricated such that they are sensitive to defectsthat occur in IC product, but are designed so that the presence ofdefects is more readily ascertained. Such defect test structures areoften constructed on the same semiconductor substrate as the ICproducts.

One example of a defect test structure is found in the Copper CMP TestMask Set designed at MIT. This test mask set is designed to quantify thedependence of the resulting copper line profile on parameters such asline pitch, line width and line aspect ratio. However, the MIT mask setis designed to be probed using conventional electrical testing in whichcurrent is passed through the device by contacting predefined pad oflarge area (approximately 100×100 μm²) with electrical probes, not byelectron beam. As is well known in the art, defect detecting systemsfrequently utilize charged particle beams. In such systems, a chargedparticle beam, such as an electron beam, is irradiated on defect teststructures. The interaction of the electron beam with features in thecircuitry generates a number of signals in varying intensities, such assecondary electrons, back-scattered electrons, x-rays, etc. Typically,electron beam methods employ secondary electron signals for the wellknown “voltage contrast” technique for circuit defect detection.

The voltage contrast technique operates on the basis that potentialdifferences in the various locations of a test structure underexamination cause differences in secondary electron emissionintensities. Thus, the potential state of the scanned area is acquiredas a voltage contrast image such that a low potential portion of, forexample, a wiring pattern might be displayed as bright (intensity of thesecondary electron emission is high) and a high potential portion mightbe displayed as dark (lower intensity secondary electron emission).Alternatively, the system may be configured such that a low potentialportion might be displayed as dark and a high potential portion might bedisplayed as bright.

A secondary electron detector is used to measure the intensity of thesecondary electron emission that originates only at the path swept bythe scanning electron beam. A defective portion can be identified fromthe potential state of the portion under inspection. In one form ofinspection, the mismatched portion between the defective voltagecontrast image and the defect free one reveals the defect location.

Thus, in such systems, the voltage contrast is simultaneously monitoredfor both defective and defect free circuits for each circuitmanufactured. However, considering the density of IC's currentlyproduced, the time necessary to scan voltage contrast data to performcomparisons is significant. The inspection and analysis of such circuitsmay take several days. Accordingly, more efficient voltage contrastinspection systems are desirable.

SUMMARY

The present invention includes a system for detecting defects in teststructures. The system operates so as to provide efficient and effectivetesting of defects. It also includes novel test structures that providefor improved defect testing, as are described more fully below.

In one embodiment, a method of inspecting a sample is disclosed. Thesample is scanned in a first direction with at least one particle beam.The sample is scanned in a second direction with at least one particlebeam. The second direction is at an angle to the first direction. Thenumber of defects per an area of the sample are found as a result of thefirst scan, and the position of one or more of the found defects isdetermined from the second scan. In a specific embodiment, the sampleincludes a test structure having a plurality of test elements thereon. Afirst portion of the test elements is exposed to the beam during thefirst scan to identify test elements having defects, and a secondportion of the test elements is exposed during the second scan toisolate and characterize the defect.

In another method embodiment, a first portion of the sample is scannedin a first direction with at least one particle beam, and a secondportion of the sample is scanned in a second direction with at least oneparticle beam. The second direction is at an angle to the firstdirection. A general location of each defect within the second portionis determined from the first direction scan, and a specific location ofeach defect within the second portion is determined from the seconddirection scan.

In another method aspect, a first portion of the sample is scanned in afirst direction with at least one particle beam, and a second portion ofthe sample is scanned in a second direction with at least one particlebeam. The second direction is at an angle to the first direction. Aninventory of defects within the second portion is determined from thefirst direction scan, and a characterization of defects within thesecond portion is determined from the second direction scan.

In yet another embodiment, a method for detecting and locatingelectrical defects in a semiconductor die present in a vacuum isdisclosed. A defect in the semiconductor die is detected. Withoutbreaking the vacuum, a focused ion beam is utilized to remove structurefrom the semiconductor die to uncover and then characterized the defect.

These and other features and advantages of the present invention will bepresented in more detail in the following specification of the inventionand the accompanying figures which illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an SEM inspection system inaccordance with one embodiment of the present invention.

FIG. 2 illustrates the typical scan pattern of the present invention.

FIG. 3 is a flow chart illustrating an inspection procedure inaccordance with one embodiment of the present invention.

FIGS. 4 a and 4 b illustrate a semiconductor wafer comprising a diearray that is prepared in accordance with the principles of the presentinvention.

FIGS. 4 c and 4 d illustrate a test die fabricated in accordance withthe principles of the present invention.

FIG. 5 illustrates in plan view an exemplary portion of the test die ofFIGS. 4 c and 4 d.

FIGS. 6 a through 6 c illustrate a part of a test structure in planview, cross section and a side view.

FIGS. 7 a and 7 b show a part of a test structure having M2 interconnectwiring and island elements at the top.

FIGS. 8 through 13 illustrate test structures that are designed to testthe integrity of covered metal layers.

FIGS. 14 a through 14 c exemplify a part of a test structure comprisingan array of via chains.

FIGS. 15 a and 15 b show an exemplary array of isolated contact teststructures.

FIGS. 16 a through 16 c illustrate a CMP pitch test pattern.

FIG. 17 illustrates a CMP density test pattern.

FIG. 18 illustrates a further CMP density test pattern.

FIG. 19 illustrates a CMP horizontal aspect ratio test pattern.

FIG. 20 shows a section of a CMP aspect ratio test pattern.

FIG. 21 shows a section of a CMP aspect ratio test pattern.

FIGS. 22 a and 22 b show top view test structures used to observemisalignment problems.

FIG. 22 c shows a cross section of the test structures of FIGS. 22 a and22 b.

FIGS. 23 a and 23 b shows an array of test structures used to observemisalignment and amount of misalignment in the y direction and xdirection, respectively.

FIGS. 24 a and 24 b exemplify a multilevel test structure converted froma metal filler for monitoring the integrity of the CMP process with theaddition of vias and/or contacts.

FIG. 25 is a diagrammatic representation of an analysis tool foranalyzing a failed test die containing test structures.

FIGS. 26 a through 26 c illustrate the process for utilizing dummyshapes for purposes of testing for defects.

FIG. 27 illustrates a product chip.

FIG. 28 provides a cross-sectional view of vertical taps for testingpurposes.

FIG. 29 shows test structures that can be included in a scanned swath.

FIG. 30 illustrates a test structure having a plurality of straight anduniform-width lines.

FIG. 31 illustrates a CMP test structure for measuring CMP linethickness.

FIG. 32 illustrates a serpentine type test structure that may beutilized to measure line resistance.

FIG. 33 illustrates test structures suitable for stepper-typetechniques.

FIG. 34 is an example of a defect that forms an edge type pattern.

FIG. 35 is a diagrammatic representation of the test structure of FIG.32 with the addition of a conductive guard ring.

DESCRIPTION OF SPECIFIC EMBODIMENT(S)

As will be further described below, the preferred embodiment of thepresent invention provides automated, rapid, contactless waferinspection capabilities in order to detect, isolate and characterizeelectrical defects impacting integrated circuits.

Several embodiments of the present invention are described herein in thecontext of exemplary multilevel integrated circuit structures, includingsemiconductor structures and overlying metallization or otherinterconnects, using various levels of conductors that are separatedfrom each other and the substrate by dielectric layers. However,structures formed using other methods of semiconductor fabrication alsofall within the scope of the present invention.

One application of the present invention includes the operation of ascanning electron microscope (SEM) with a continuously moving stage.However, the test structures and many of the methods described hereinare also useful in the context of other testing devices, including SEM'soperated in step and repeat mode. As an alternative to moving the stagewith respect to the beam, the beam may be moved by deflecting the fieldof view with an electromagnetic lens. Alternatively, the beam column maybe moved with respect to the stage.

A. Utilization of Scanning Electron Microscope With Continuously MovingStage for Scanning Primary Area

The present invention, in one embodiment, utilizes an SEM with acontinuously moving stage. Use of such an SEM provides significantadvantages in connection with the detection of defects in semiconductordevices, as is described more fully below.

An SEM system may be used to provide automatic inspection of wafers andother substrates. Such SEM systems are well known in the art. Forexample, U.S. Pat. No. 5,578,821, issued to Meisburger et al. on Nov.26, 1996 and entitled “ELECTRON BEAM INSPECTION SYSTEM AND METHOD,”describes an apparatus for a charged particle scanning system and anautomatic inspection system, including wafers used in microcircuitfabrication. This patent is herein incorporated by reference in itsentirety. In the Meisburger apparatus, a charged particle beam isdirected at the surface of a substrate for scanning that substrate and aselection of detectors are included to detect at least one of thesecondary charged particles, back-scattered charged particles andtransmitted charged particles from the substrate.

The substrate is mounted on an x-y stage to provide at least one degreeof freedom in movement while the substrate is being scanned by thecharged particle beam. The substrate may also be subjected to anelectric field on or near its surface to accelerate the secondarycharged particles. The system facilitates inspection at low beamenergies on charge sensitive insulating substrates and has thecapability to accurately measure the position of the substrate withrespect to the charged particle beam.

Additionally, there is an optical alignment system for initiallyaligning the substrate beneath the charged particle beam. To functionmost efficiently there is also a vacuum system for evacuating andrepressurizing a chamber containing the substrate. The vacuum system canbe used to hold one substrate at vacuum while a second one is beingloaded/unloaded, evacuated or repressurized. In the inspectionconfiguration, there is also a comparison system for comparing thepattern on the substrate with a second pattern.

The '821 patent further describes an automatic system for the automaticinspection of a substantially non-conductive substrate. The systemincludes a field emission electron source to provide an electron beam, acharged particle beam column to deliver and scan the electron beam fromthe field emission electron source on a surface of the substrate, one ormore charged particle detector(s) to detect one or more of three typesof charged particles emanating from the top and bottom surfaces of thesubstrate, (namely, secondary charged particles, back-scattered chargedparticles and transmitted charged particles). The system also includes acontinuously moving x-y stage disposed to receive the substrate and toprovide at least one degree of motion to the substrate while thesubstrate is being scanned by the charged particle beam and amulti-processor image defect computer coupled to the charged particledetector(s) to identify defects on the substrate. Such a system issuitable for practicing the preferred embodiment of the presentinvention.

Similarly, U.S. Pat. No. 5,502,306, issued to Meisburger et al. on Mar.26, 1996, entitled “ELECTRON BEAM INSPECTION SYSTEM AND METHOD,”describes an inspection system suitable for practice of the presentinvention. In the '306 patent, numerous embodiments of a method andapparatus for a particle scanning system and an automatic inspectionsystem suitable for practice of the preferred embodiment of the presentinvention are described. Applicants incorporate herein this referenceU.S. Pat. No. 5,502,306 in its entirety.

The inspection system utilized in the practice of the present inventioncan operate in several modes, including, for example, array, die-to-dieand die-to-database. In each of these modes, defects are detected bycomparing an electron beam image derived from scanning the substrateagainst a standard. In array mode, signals from a first portion of anarray of substantially identical circuit elements are compared withsignals from a second portion of such an array. In a variation on thistechnique, an image of the array can be compared with an electronicallymodified version of the same image, and the repeating content can besubtracted. A resulting difference image will show the defects. Anexample of such a system is shown in commonly assigned U.S. Pat. No.5,537,669, issued to Evans et al. on Jul. 16, 1996, entitled “INSPECTIONMETHOD AND APPARATUS FOR THE INSPECTION OF EITHER RANDOM OR REPEATINGPATTERNS,” which patent is incorporated herein by reference in itsentirety.

In die-to-die inspection, signals from two die of the same substrate arecompared with each other. In die-to-database inspection the signal fromone die derived from the electron microscope is compared with a signalthat is derived from the database. The database may include design datathat is used to make the die and to generate a plurality of perfectimages of how the die would appear without any defects. For example,each image contains voltage contrast signatures specifying how thecorresponding test image should appear without defects. The perfectimages are compared to corresponding images obtained from the die.Alternatively, the database may include the plurality of perfect imagesthemselves. In the case of die-to-die inspection, the function of thedefect processor is to compare image data obtained from a first die withimage data obtained from a second die, or, in the case ofdie-to-database inspection, to compare image data obtained from a diewith data derived from a database adapter. Preferably, the defectprocessor is a multi-pixel image computer that allows efficientcomparisons to take place. In a specific embodiment, the processoroperates with pixel resolution sizes in a range of about 25 nm to 2000nm. In more general terms, the processor operates with a pixel sizenominally equivalent to two times a width of the test structure linewidth to maximize throughput at optimal signal to noise sensitivity. Theroutines and the basic implementation of a defect processor aredescribed in U.S. Pat. No. 4,644,172, issued to Sandland et al. on Feb.17, 1987, entitled “ELECTRONIC CONTROL OF AN AUTOMATIC WAFER INSPECTIONSYSTEM,” which patent is incorporated herein by reference in itsentirety. Other inventive inspection techniques (e.g., die-to-truthtable and die-to-perfect image) are also described below.

The sample to be inspected may be supported by a holder that is placedbeneath an electron beam column on an x-y stage. The sample should bealigned on the stage such that the x-directional motion of the stage issubstantially parallel to the x-axis of the core area of the samplepatterns, i.e., the area of interest for the inspection. Once the sampleis properly aligned, the inspection process is initiated.

The column and an analog deflection circuit direct an electron beamtowards the sample surface, and the detector(s) may detect one or moreof the secondary electrons, back-scattered electrons, and transmittedelectrons. The position and movement of the stage during the inspectionof the sample is controlled by a stage servo.

FIG. 1 provides an overall block diagram of an inspection system 10suitable for practice of one embodiment of the present invention. Insystem 10 an automatic inspection apparatus of X-ray masks, wafers, andother samples, is shown which uses a scanning electron microscope as itssensor.

Sample 57 to be inspected is held in a holder which is automaticallyplaced beneath electron beam column 20 on x-y stage 24 by sample handler34. This is accomplished by commanding sample handler 34 by systemcomputer 36 to remove the sample 57 of interest from a cassette with theflat or notch (see FIG. 4 a) on sample 57 being detected automaticallyto properly orient the sample 57 in handler 34. The sample is thenloaded under column 20. Next, the operator visually observes the maskthrough optical alignment system 22 to locate the alignment points onthe sample (these may be any operator selected features on the sample)to ensure that the x-directional motion of the stage is substantiallyparallel to the x-axis of the care area of the sample patterns, i.e.,the area of interest for the inspection. That completes the coarsealignment.

Fine alignment is subsequently achieved by the operator scanning thesample with the electron beam and observing the image on image display46. All alignment data is then stored in alignment computer 21 whichworks in cooperation with system computer 36 for calculation of theactual combined x and y motions necessary to scan the die along its xand y axes so that no further operator alignment action is required forinspections of the same type of samples. Once the sample is properlyaligned, the inspection process is initiated.

Column 20 and its optical alignment system 22 and analog deflectioncircuit 30 (as described more completely below) then direct an electronbeam towards sample surface 57, and detectors 32 detect the secondaryelectrons, the back-scattered electrons and those electrons which passthrough sample 57. That operation and the data collection from thatexposure is performed by column control computer 42, video frame buffer44, acquisition pre-processor 48, deflection controller 50, memory block52. Bus VME1 29, serves as the communication link between thesubsystems.

The position and movement of stage 24 during the inspection of sample 57is controlled by stage servo 26 and interferometers 28 under the controlof deflection controller 50 and alignment computer 21.

When the comparison mode is die-to-database, database adapter 54 incommunication with memory block 52 is used as a source of the signalthat is equivalent to the expected die format.

The actual defect processing is performed on the data in memory block 52by defect processor 56 in conjunction with post processor 58, with thecommunication between these blocks being via bus VME2 31.

The overall operation of the system is performed by system computer 36,user keyboard 40 and computer display 38 in communication with the otherblocks via a data bus 23 which may be similar to an Ethernet bus.(Ethernet is a trademark of Xerox Corp.).

In one embodiment of the invention, significant benefits are achieved byscanning with a continuously moving stage. That is, measurements of thesample are obtained while the stage (or beam) is moving. In contrast,stepper type systems utilize alternating cycles of movement andmeasurement of the sample. Additionally, stepper type systems requiretime to settle after each movement before measurements of the sample canbe taken. The illustrated embodiment provides a more efficientmechanisms for taking measurements during movement of the sample withoutrequiring any settle periods.

In the illustrated embodiment, the stage moves continuously in thex-direction. More preferably, the stage moves continuously at a constantspeed in the x-direction. Typical speeds of the continuous movement ofthe stage are approximately 1.0 to 200 mm per second. Note that movementin the x-direction can also be achieved by movement of the e-beam,whether by movement of the actual e-beam column or by deflection of thebeam rather than the stage. Moreover, the stage itself can operate suchthat it moves in both the x- and y-directions or a combination thereof.

Simultaneously with the continuous movement of the stage in thex-direction, the electron beam is repeatedly deflected back and forth inthe y-direction. During a typical application of the present invention,the e-beam may move back and forth at approximately 100 kHz. Preferably,the deflection is largely free of distortion and is substantiallyperpendicular to the surface, so that the imaging characteristics areuniform over the scan field.

FIG. 2 illustrates a scan pattern. Here a single test chip 100 is shownon a substrate. Within the test chip there is a scanned swath (or “areaof significance” or “primary scan area”) 101 that is to be inspected.During the inspection of the die, the effective scanning motion in thex-direction is provided by a moving stage and the effective motion inthe y-direction is provided by deflection of the electron beam.

As a result of the combined movement of the stage and the e-beam, thepath of the beam relative to the substrate forms a scan pattern 102 asis shown in FIG. 2. Although this scan pattern 102 is shown as asinusoidal pattern, it could instead be a triangular pattern or othershape, and data could be gathered from the resulting secondary electronsor other emissions as the beam is scanned in either direction, or inboth directions. In the illustrated embodiment, the length of thescanned swath is the width of the test chip (e.g., 7 mm for a 7 mm by 7mm test chip or 10 mm for a 10 mm by 10 mm test chip). However, thescanned swath can be less than the width of the test chip and still fallwithin the scope of the present invention. Preferably, the width of thescanned swath is as large as possible. For current commerciallyavailable systems the scan swath is between 50 μm and 500 μm in width.More preferably, the scanned swath is approximately 200 μm.

In one embodiment, the scanned swath includes at least part of each teststructure on the test chip. So that a single scan may test all of thetest structures. The scanned swath can contain a variety of differenttypes of test structures. This permits the system to detect differingtypes of defects with a single pass through one scanned swath. However,it is also possible to create a structure having multiple swaths whereeach swath has only one type of test structure wholly or partiallytherein. The various types of test structures that can be includedwithin the scanned swath are described more fully below.

FIG. 3 is a flowchart illustrating a process and test procedure inaccordance with one embodiment of the present invention. Initially 3, inoperation 1, a sequence of initial manufacturing process steps areperformed to form a test structure such as conductive lines on asubstrate. In operation 2 the structure is inspected by an electron beaminspection system. In operation 3, detected signals from the electronbeam inspection are processed to determine whether a potential defectsuch as an open or a short in the conductive lines has been detected. Ifpotential defects of sufficient severity to warrant termination of themanufacturing process have not been detected, then in operation 4 asubsequent manufacturing process step is performed. It is not unusualfor thousands of potential defects to be found during inspection;however, they are often of insufficient number and severity to warranttermination of the manufacturing process, even for product wafers. Ifthe process is still undergoing characterization, and the wafers are notproduct wafers, the processing of such wafers and the test structuresthereon can continue even if numerous severe defects are located.

The number of test structures present in a lot of wafers and therelative area required for the test structures can be varied in a numberof ways. One possibility is to have several reticles, each with varyingamounts of area dedicated to test structures, which may be used in themanufacturing process. Reticles having a relatively large area devotedto test structures that can be used for certain wafers in a lot (or canbe stepped over for a predetermined fraction of such wafers), whilereticles having relatively fewer test structures can be used for otherwafers in the lot (or stepped over a remaining fraction of such wafers).The relative extent of usage of each type of reticle determines thenumber of test structures in the lot, and this fraction can be variedfrom lot to lot depending on the testing requirements of the process atthe time each lot is fabricated. Alternatively, a portion of a reticlecontaining test structures can be “bladed off” to a sufficient extent tocreate the desired predetermined number of test structures, and theremainder of each wafer would be covered with product structures.

If there is no subsequent manufacturing process step, the manufacturingprocess is finalized at 5. If there remains a subsequent manufacturingprocess step, the next manufacturing process step is initiated at 6.

However, if, in operation 3 a potential defect is detected, in operation7 the location of the potential defect can be recorded. In operation 8,the potential defects can be re-located on the substrate and can becharacterized using various characterization techniques such as scanningelectron microscopy, optical microscopy, Energy Dispersive X-RaySpectroscopy (EDS) and/or Focused Ion Beam (FIB) techniques, or anycombination thereof. Finally, in operations 9 and 10 information fromthe characterization process is analyzed and the resulting data can beused to eliminate defect-causing process conditions. Of course, if thetesting defect characterization process is nondestructive, the substratecan be returned to the process line for further processing.

C. Test Chip Design

Scanning a test chip using an apparatus with a moving stage provides afast and efficient method for testing semiconductor devices for defects.It should be noted, as is described more fully below, that in order tobe tested, a test structure need not reside wholly within the scannedswath. For many structures, voltage contrast testing can be accomplishedby scanning only a small portion of a test structure. In such a case, ifa defect is detected via the voltage contrast technique, the exactlocation of the defect can be determined in a subsequent operation, evenif the defect is located outside the scanned swath.

As is discussed more fully below, a myriad of different types of teststructures can be fabricated within the scanned swath. Some of thesetest structures are described more fully below. Such structures caninclude via chains and conductive lines, which preferably (but notnecessarily) reside within the scanned swath only in part. The preferredstructure of the via chains and conductive lines is described in moredetail herein. Such structures can also include more compact testelements such as contact arrays and elements designed to detect defectscaused by specific process steps, such as chemical mechanical polishing.These more compact elements preferably (but not necessarily) resideentirely within the scanned swath.

It should be further noted that the test structures described herein canalso be tested by techniques other than that described herein as thepreferred embodiment. For example, such test structures might be testedby a particle beam without a continuously moving stage, e.g. a step andrepeat type stage in which the electron beam scanning is accomplishedwhile the stage is stationary, and the stage is then moved and allowedto settle before a subsequent electron beam scanning step takes place.One embodiment of a stepper type test structure is described furtherbelow with reference to FIG. 33. Also, voltage contrast techniques thatdo not involve scanning with a particle beam can also be used inconnection with many of the test structures described herein. Forexample, a photon beam (rather than an incident electron beam) could beused to induce voltage contrast. The photon beam could be used underconditions suitable for photo electron emission microscopy (“PEEM”).

D. Exemplary Test Chip

Described in this section is an exemplary test chip that is specificallydesigned to take full advantage of the present invention. It should berecognized, however, that the specific design described herein isexemplary only and that many other designs or configuration are possiblewithin the scope of the present invention.

Several embodiments of the present invention are described herein in thecontext of exemplary multilevel integrated circuit structures, includingsemiconductor structures and overlying metallization or otherinterconnects, using various levels of conductors that are separatedfrom each other and the substrate by dielectric layers. As is well knownin the art, in such multilevel structures, a first conductor (M1) and asecond conductor (M2) can be connected by vias formed through aninterlayer dielectric (ILD). Similarly, contacts can be formed betweenthe conductors and the substrate. The defect detection system of thepresent invention is advantageously able to detect opens, intra-layershorts (within M1 or M2) or interlayer (between M1 and M2) shorts causedby errors occurring systematically or randomly during the manufacturingprocess.

In addition, specific embodiments of the present invention are also ableto detect defects caused by particular process steps, such aslithographic steps, dry etch steps, deposition steps, or a chemicalmechanical polishing (CMP) process. As is well known in the art, the CMPprocess is often used to planarize structures that build up duringmultilevel deposition processes. These structures can be used asdamascene interconnects, conductive plugs, or for other purposes. TheCMP process is expected to become even more important as thesemiconductor industry shifts to copper metallization, since coppercannot be easily dry-etched (the etch products being non-volatile), butis readily processed using CMP. However, the CMP process may polish awayfunctioning circuit parts through dishing (leading to opens) or coppersmearing (leading to shorts) when the circuit layout changes drasticallyin density, pitch and or in the horizontal aspect ratio (length: width).The defects created by CMP process are preferably detected during theinspection process.

FIGS. 4 a and 4 b illustrate a semiconductor wafer 200 comprising a diearray 202 that is prepared in accordance with the principles of oneembodiment of the present invention. As illustrated in FIG. 4 b, thearray 202 may be comprised of a plurality of test dies 204 and aplurality of actual product dies 206 containing intended integratedcircuitry. As will be described below, the test dies 204 allow defectlocation, defect identification, defect type and defect density throughin-line inspections during the actual manufacturing process ofintegrated circuits.

The ability to detect defects in line (i.e., during the manufacturingprocess) is a significant advantage of the present invention. Unlikefunctional testing of semiconductor devices performed on completedwafers by wafer probing methods, the present invention can performtesting in line. This results in better and more timely information forthe engineers controlling the manufacturing the process, and providesthem with an opportunity to service machinery or alter processconditions to improve yield, before many devices are lost as scrap. Bycontrast, if the engineer were to wait until processing of the deviceswere completed, millions of dollars may be lost due to poor yieldbecause of the delay. Moreover, the test methodologies of the presentinvention could be used as part of an Advanced Process Control (“APC”)system, in which data from the testing process is fed to automatedcontrol systems that improve process yield with little or no humanintervention, based on software algorithms that take into account theequipment and process technology used in the manufacturing process. Asan example, test structures designed to detect CMP overpolish couldprovide data that automatically feeds back to the CMP process and causesthe process to be modified such that polishing time is reduced, orpressure on the polishing pad is diminished.

The test dies 204 may be laid out in an orderly row-column fashion asshown in FIG. 4 b. Such an orderly layout can be also used as a map toevaluate final test data and to locate certain types of defectsoccurring on various locations on the wafer during the manufacturingprocess. In one embodiment, the test dies 204 occupy a predeterminedarea on the wafer 200 so as to provide a statistically significant dataset of test data, yet minimize the inspection cost and avoid adverselyaffecting the production yield. Such a test die has about the samedimensions as the product die in the range (e.g., 10 mm by 10 mm or 7 mmby 7 mm). Alternatively, when developing a new process, the wafer couldbe comprised entirely of test dies.

As illustrated in FIG. 4 c, each individual test die 204 may be squareor rectangular in shape defined by first, second, third and fourth edges204A-204D, and configured to have a number of portions, namely, a firstportion 206 and a second portion 208 separated by an intermediateportion 210. Sections 206-210 define the test sites where the teststructures are formed. The portions 206, 208 and 210 of the test die 204are preferably rectangular in shape.

As shown in FIG. 4 c, the first portion 206 may be defined by a proximaledge 206A, a distal edge 206B, a first edge 207A and a second edge 207B.The second portion 208 may be defined by a proximal edge 208A, a distaledge 208B, a first edge 209A, and a second edge 209B. Finally, theintermediate portion 210 may be defined by the distal edges 206A and208A, a first edge 210A and a second edge 210B.

As will be described more fully below, the test structures representstructures constructed in various steps of the semiconductor ICmanufacturing process. Once the portions 206-210 are defined on the testdie 204, the portions 206-210 may also be further divided into aplurality of sections, preferably rectangular, on which test structuresmay be placed by test type, as in the manner shown in FIG. 4 c.

In a specific embodiment, the first portion may comprise a first section212, a second section 214 and a third section 216. Similarly, the secondportion 208 may be divided into three sections: namely a fourth section218, a fifth section 220 and a sixth section 222. As will be describedmore fully below, each of the sections 212-222 as well as the portion210 may comprise one or more groups of test structures that are laid outin a predetermined fashion according to their design (i.e., the type offunctional structure to which the test structures correspond, or thetype of defect the test structure is designed to find). The quantity oftest structures of each type in a given portion, or on a test chip, orover an entire wafer, is selected to provide a statistically significantsample of such test structures, so that a statistically significantquantity of defective regions is likely to be found therein. Thisquantity will vary with the expected yield of the chips on the wafer.For example, in a low yield process (as might be expected to be found ina process that is under development and not yet in production), arelatively small number of test devices might be needed to obtain areasonable sample size. By contrast, in a high yield process, a largersample size (and therefore more test structures) may be needed. Thequantity of test structures may also be varied depending on the goals ofthe chip manufacturer. For example, if the chip manufacturer isunwilling to devote much space on the wafer to test structures, thenumber of such structures could be kept relatively small. If processproblems arise, the quantity of test structures present on each wafercould then be increased (to provide the manufacturer with more test dataregarding the process) until the nature of the problem has beendetermined and corrected.

Data from such testing can also be used by both fabless integratedcircuit makers, and the foundries that do their manufacturing. Forexample, the test structures may be inspected during the manufacturingprocess by the foundry. The same test structures may then bere-inspected by the fabless IC company before acceptance, to determineif the lot of wafers is likely to have an acceptable yield, to have anacceptable result during reliability testing, and to have acceptablequality. The data can also be used by the fabless IC company to improvethe designs of its integrated circuits to make them easier tomanufacture at higher yield and/or with superior quality. This data maybe used as part of the purchasing process, so that the fabless companiescould base their payment to the foundry, at least in part, on statisticscalculated from the testing process, such as the predicted yield. Forexample, if the payment were to be made based on predicted good die, alot having 500 die, and a predicted yield of 80 percent based on voltagecontrast testing, could result in a payment for 400 predicted good dieto the IC manufacturer. Testing by the fabless company could befacilitated, as described in more detail below, by using stacked viascontaining stacked conductive plugs to create vertical conductive pathsthat tap down to the lowest test structures. Such test structures wouldotherwise be rendered inaccessible by subsequent processing, which wouldbury them under insulators and make them impossible to see in anelectron beam system. However, by adding one or more conductive taps,these buried features can be re-tested even after all levels of theentire wafer have been fabricated. This makes the wafer testable byfabless semiconductor companies, and permits them to verify test dataprovided by the foundry. As described below, probable pads may also beadded to the described voltage contrast test structures so that standardwafer probing techniques may also be implemented without the need for anexpensive SEM device, for example.

Data from such testing could also be used to determine the need forother sorts of tests. For example, reliability is a relatively timeconsuming and expensive test. By processing data derived from the teststructures of the present invention, one can better predict whetherreliability testing is needed and how many chips from each lot should besubjected to such testing. For example, certain reliability failuremechanisms may be indicated as likely when test data from at leastcertain types of the test structures of the present invention indicatesthat defects are present which are close to, but not quite reaching, aquantity or level of severity that would affect device functionality.Under such circumstances, one could predict that reliability testingconditions would cause such devices to fail at an unacceptable level.Reliability testing could be conducted to verify this, and the process(if desired) could be altered to lessen the severity of such predictedreliability failures, even before the reliability testing is actuallyperformed.

As previously mentioned above, the first section 212 may comprise teststructures involving M1 interconnect level, for example. A teststructure formed on the first section 212 may represent the M1processing stage of an integrated circuit and allow testing andevaluation at this stage of the production. At this stage, inspectionsmay be performed to detect opens and intra-layer shorts in the M1interconnect wiring, in-line, during the manufacturing process.Similarly, test structures relating to M2 and M3 interconnect lines areformed on the third and fifth sections 216 and 220 respectively. M2 andM3 test structures also allow detection of opens and shorts, andinterlayer shorts such as those between the M1 and M2 interconnect teststructures. Via chain test structures may be formed on the second,fourth and sixth sections 214, 218 and 222. The second section 214, forexample, may represent via chains constructed between M1 and M2interconnect lines. In this respect, the fourth and sixth sections 218and 222 may have via chain test structures representing vias between M2and M3 interconnect lines and M3 and M4 interconnect lines,respectively.

As will be described below, via chain test structures allow two types oftests, namely opens only tests to detect opens in via chains, and opensand shorts tests to detect both opens in the chain and shorts betweenthe neighboring conductor or metal shapes.

In this embodiment, various other groups of test structures are formedalong the intermediate portion 210 of the die 204. As is described morefully below, such test structure groups may comprise CMP teststructures, overlay or misalignment structures and individual contacts,as well as dummy CMP fillings. In this embodiment, dummy CMP fillingscan also be formed on available unoccupied areas on the product dies,such as those available along the corners of the die or unoccupied areasfound between the neighboring circuit layout sections.

As illustrated in FIG. 4 d, sections 212-222 may be further divided intoa plurality of subsections or modules to have test structures withdiffering critical dimensions over such modules. In this context,critical dimension often refers to a predetermined test size of afeature or the distance between features. By way of using teststructures having different critical dimensions, superior IC featurescan be produced. In other words, if a particular acceptable criticaldimension produces the best result with respect to a specific designfeature, that critical dimension can be adopted and design rules forthat process or product family can be altered accordingly. Moreimportantly, test structures implemented with various geometries andcritical dimensions help to determine which device geometries ordimensions are more prone to certain defects, and can predict theresulting defects occurring between the two devices due to the distancebetween them.

In this respect, for example, the M1 interconnect lines in the section212 may be formed on a first, second, third, fourth and fifth modules212A-212E, each module having one type of test structure group formedwith a critical dimension different than the critical dimensions used inother modules. The modules 212A-212E help to determine which criticaldimension for M1 interconnects are more vulnerable to opens, shorts andinterlayer short type of defects. Similarly, first, second, third,fourth and fifth modules 216A-216E in section 216 and first, second,third, fourth and fifth modules 220A-220E in section 220 provideinterconnect lines with differing critical dimensions for M2 and M3process steps respectively, for opens, shorts and interlayer short typesof defects.

Referring to FIG. 4 d, the section 214 may be comprised of two modules,namely first and second modules 214A and 214B, each comprising via chaintest structures formed between the M1 and M2 interconnect levels. Inthis embodiment, first module 214A may comprise via chains that areformed to detect opens type of defects while the second module 214Bcomprises via chains to detect both opens and shorts type of defects. Aswill be described more fully below, in the second module 214B,individual via chain lines may be interposed neighboring metal lines soas to detect and monitor both shorts occurring between the via chainsand the neighboring metal lines and opens in individual chains.Similarly, first and second modules 218A and 218B in section 218 andfirst and second modules 222A and 222B in the section 222 provide viachain test structures to monitor and detect opens as well as opens andshorts types of defects as described for modules 214 a and 214B.

It should be noted again that the above-described test chip is exemplaryonly. For example, test structures can be located in different places insuch a chip, or certain test structures described herein may not beincluded at all. Many different configurations are possible within thescope of the present invention.

E. Testing of the Exemplary Test Chip

As previously described, an electron beam inspection system, such as anSEM, may be used. When the electron probe strikes over the surface ofthe die at a given point, this action gives rise to signals that can becollected by the detector to give information about that point. Theelectron beam system is programmed to move the wafer to bring the firsttest chip into position at a low magnification (60-500×), and locate,for example the first line of features such asopens/shorts-test-patterns in a linear array, and draw the electron beamscan line through them in a raster mode.

In one embodiment, the primary signal source comprises electrons.Secondary electrons are generated as a result of the incident (primary)electron beam striking the surface of the wafer. The secondary electronsare emitted from the wafer and collected by the detector to create theimage. In the image, the high-intensity secondary electron emittingfeatures come out, for example, visually brighter than the low-intensitysecondary electron emitting features. If this variation in secondaryelectron emission intensity is plotted with respect to the distance,along the swath-length that the electron beam scans, an intensitydistribution of the scanned test structure is obtained.

In a die-to-database or die-to-perfect-image inspection mode, the systemoperates to match the intensity distribution of a defect-free teststructure as represented in the data base to the intensity plot of thecorresponding structure on the test chip provided by the electron beamunit. In one embodiment, the die under test is compared to an image thatrepresents what the die under test looks like without any defects. Forexample, an array of interconnect lines may alternate between being tiedto ground and left floating. In this case, a perfect image is generatedthat has alternating dark and bright interconnect lines. This comparisonoperation allows the detection of all the defects in the test structurein terms of missing and/or extra peaks. Alternatively, the intensitypeaks of the test structure may be compared to a predetermined set ofexpected values, e.g., arranged in a truth table. For example, a set ofinterconnect lines on the test structure under test may be expected tohave alternating high and low intensity values. In one embodiment, eachline is compared with a predetermined threshold to determine whether theconductive line is grounded or floating. These high and low values arethen compared to the expected high and low values. The test structureitself may also be configured to facilitate die-to-perfect dieinspection and die-to-truth table inspection. In one embodiment, thelength of the proximal-ends (or stubs) of the test pattern have varyinglengths, which may also be matched to the corresponding perfect dieportion or truth table values. For example, two stubs in a row may havea same length (while the other stubs have a different same length) toindicate a starting or reference point to begin the comparisonprocedure. In a second example programmed defects are incorporated atfixed intervals along the scan path to form a fixed grid to aid indefect location during subsequent analysis.

The generated perfect image and/or set of predetermined intensity valuesor truth table values, along with the corresponding test structures, mayalso be provided to the customer so that they may easily inspect thecorresponding test structures. Of course, standard die-to-die and arrayinspection techniques may also be utilized. However, thedie-to-database, die-to-perfect image, and die-to-truth-table techniquesrepresent a more efficient inspection procedure since a perfect die ordie portion does not have to be found for comparison to the die undertest.

The scanning electron microscope, in the voltage contrast mode, enablesone to distinguish the charged floating conductor shapes from thecharge-drained grounded shapes in terms of visual or intensitycontrasts. These shapes can be monitored visually on a CRT-screen, or,preferably, stored and analyzed electronically. This principle has beenused previously to manually locate, isolate, and in-situ characterizedefects causing unintended discontinuities to ground or unintendedshorts to the neighboring grounded-shapes in a product chip. However,such a manual product-inspection process is extremely tedious and slowbecause of the complexity of the product design and the highmagnification source on the poorly contrasting CRT screen of the SEM.

In FIG. 5, in plan view, an exemplary portion 400 of the test die 204 isshown in detail. The portion 400 gives a detailed view of parts of thefirst portion 206, the second portion 208 and the intermediate portion210 of the test die 204 (e.g., of FIG. 4 c). One technique of thepresent invention will be exemplified using first test structures 402 onthe first portion, second test structure 404 on the second portion ofthe test die 204 and third test structures 406 on the intermediateportion of the test die 204. It is understood that, during the electronbeam inspection of the test die 204, the electron beam is scanned overthe intermediate portion in a raster mode and in the direction of arrowA. The electron beam interacts with the test structures on theintermediate portion as well as with the proximal ends (referred to as“stubs”) of the test structures on the first and second portions butlocated adjacent to proximal edges of the first and second portions. Inthe following section of this application, the first, second and thirdtest structures are described in further detail.

As is described more fully below and seen in FIGS. 5, 6 a, 6 b, 6 c, thefirst test structures 402 comprises rows of first conductors 408 andsecond conductors 409, extending parallel to the edges 204B and 204D,and between the proximal and distal edges 206A and 206B of the test die204 (See FIG. 4 c). The second conductors 409 are located between therows of first connectors 408 as in the manner shown in FIG. 5. In aspecific embodiment, proximal stub ends 410 of the first connectors 408,which are referred to herein as interconnect lines, are located at theproximal edge 206A of the portion 206 of the test die 204 (See FIG. 4c), and distal ends 412 of the interconnect lines 408 are located at thedistal edge 206B of the die portion 206. It is understood that the firsttest structure 402 exemplified in FIG. 5 may represent a plan view ofany of the metal interconnect layers such as M1, M2, M3 or M4. As willbe described below, the distal ends 412 of the interconnect lines 408are grounded to the substrate of the die 204 while the proximal stubends 410 are free and are not grounded to the substrate. As is describedbelow, the second connectors 409, which are referred to as islandmembers, are not grounded and help to distinguish defects occurring ingrounded interconnect lines 408 in the voltage contrast mode. Theseproximal ends of 408 and 409 may be extended to different lengths forexposure in the primary-scan area, as described earlier. Also, it willbe understood that 408 and 409 shapes can be straight-lines withoutcorners or islands as shown in FIG. 32.

In a preferred embodiment, each stub 410 has a width that is the same orless than the rest of the interconnect line 408. That is, a widened flagarea is not utilized. Since the stub width is the same or less than therest of the interconnect line, a plurality of interconnect lines may bedensely packed in a simplified array. In other words, the proximal endof the test structure itself can be scanned “as is” in the Primary-ScanArea. In this embodiment, the spot size of the inspection SEM isgenerally configured to the stub's dimensions.

Referring to the exemplary test chip illustrated in FIGS. 4 a-d, thetest chip is preferably scanned in a single pass. The scanned swathpreferably includes all of intermediate portion 210. Further, portion210 is preferably located substantially in the center of the test chip.Additionally, in one embodiment, the intermediate portions 210 of aplurality of test chips are aligned such that a single scan may beperformed on a plurality of intermediate portions 210 on a plurality oftest chips.

Preferably, the scanned swath includes an area larger than portion 210,however. In a specific embodiment, all of the test structures on thetest chip are scanned in a single pass. Therefore, where the width ofthe scanned swath is 200 μm, for example, the width of portion 210 mightbe 190 μm. In such an instance, the scanned swath might include up to 5μm above and up to 5 μm below portion 210.

In such an instance, it would be possible, and preferable, to inspectall of the test structures of the test chip with only a single scan ofthe scanned swath (e.g., the Primary-Scan Area). In this regard, a scanof the scanned swath would scan all the test structures that reside inportion 210. Such a scan would also scan portions of the test structuresof portions 212, 214, 216, 218, 220, and 222. In the event the scan ofthe scanned swath reveals a defect in one of the test structures ofportions 212, 214, 216, 218, 220, or 222, the system, according to aspecific embodiment, would then take steps to locate and furthercharacterize the defect (which is typically located outside the scannedswath (e.g., in the potential Secondary-Scan Area).

It should be noted that the scanned swath need not be located in thecenter of the test chip, but can be located elsewhere on the chip. Forexample, the scanned swath could be located at the bottom or top of thechip. In the case, for example, where the scanned swath is located atthe bottom of the chip, the scanned swath might comprise test structuresthat reside entirely within the scanned swath, as well as portions oftest structures that extend upward from the scanned swath.

Further, a test chip might be designed to have more than one scannedswath. In an alternative embodiment, there are multiple intermediateportions 210 within a single test die so that the aspect ratio of eachtest structure is optimized. For example, the interconnect lines 408 maybe shortened so as to reduce their resistance, capacitance and/or thetime required to locate a defect on such lines. In another embodiment,intermediate portions (e.g., 210) and corresponding first and secondportions (e.g., 206 and 208) are configured in an array so that scans ofthe intermediate portions may be conducted in an array mode. Inaddition, a scanned swath might not be coextensive with the width of thetest chip, but might be located entirely on the inside of the test chip.In fact, it is possible to have several scanned swaths on a single testchip, all located entirely inside the chip. However, it should berecognized that it is preferred that any scanned swath comprise specifictest structures that reside entirely within the scanned swath andother-specific test structures that reside only in part within thescanned swath.

F. Conductive Line Test Structures

One type of test structure that can be included on a test chip may haveits end-regions projecting into the Primary-Scan-swath. In the preferredembodiment, such test structures will reside within a scanned swath onlyin part. It is preferable only to scan initially a small portion of aconductive line. If a defect is detected, then further testing andanalysis can be undertaken in order to locate the defect more preciselyand to characterize it.

FIGS. 6 a-6 c illustrate a part of the test structure 402 an M1 wiringtest structure in plan view, cross section and a side view in detail.Referring to FIGS. 6 a-6 c, the M1 interconnect 408 and thefloating-member 409 are formed over a substrate 420 using conventionalsemiconductor IC process techniques such as Chemical Vapor Deposition(CVD), patterning and etching techniques. An isolation layer 421, suchas an oxide layer, that is interposed between the substrate 420 and theM1 interconnect 408 isolates the M1 interconnect 408 from the substrate420. The island member 409 is separated from the M1 interconnect by adielectric layer 422 and configured such that the island member 409 hasan elevated posture as in the manner shown in FIG. 6 c in side view. Theinterconnect line 408 is connected to the substrate 420 through acontact that may be etched through the oxide layer 421. Through thecontact 424, the interconnect line 408 is electrically grounded to thesubstrate 420.

An electron beam may scan across the distal end 410 of the exposedmetals of the interconnects 408 and floating-members 409 to detectdefects, for example, opens, shorts and intra-layer shorts in the M1wiring. This preferred embodiment provides an immediate calibration ofthe scan-intensities of the grounded or floating shapes during thePrimary Scan. Detection of intra-layer shorts and opens tests areperformed by scanning the electron beam across the distal end 410.

During the electron beam inspection, since the island members 409 arenot grounded, electron flow from the incoming incident electron beamcharges island members 409. Contrary to the grounded interconnect lines408, electrons cannot find a path to the ground and secondary electronemission does not occur, as a result the island members 409 charged bythe beam remain nominally dark. Owing to their ground connection andassuming that they are defect free, however, the interconnect lines 408remain charge-drained and generate significant amount of secondaryelectron radiation to be detected by the detector of the electron beamsystem. For the grounded interconnect lines 408, the electrons from thebeam will find a path to ground and secondary electrons will be emittedfrom that particular interconnect line 408 if such interconnect line 408is not broken. As a result, that interconnect line 408 will appearbrighter or appear to glow, thus indicating that the correspondinginterconnect line does not have an open (i.e., it is not defective). Theintensity peak arising from this secondary electron emission will alsobe registered by the system to match with the corresponding data on thisparticular test structure. However, if the line under test is broken(i.e., it is open), there will be less secondary electron emission and,thus, the interconnect 408 will remain dark. As previously mentioned,the missing peak will then be registered by the system, hence indicatinga potentially defective interconnect for further analysis. In anotherpossible case, if an interconnect line is shorted to anotherinterconnect line for the same M1 wiring, both interconnect lines 408and 409 will glow, indicating the electrical short between them.

With the inventive test die, the quality of the M1 wiring or otherlevels of wiring can be monitored throughout the manufacturing process.During the manufacture of multi-level metal structures, elevatedtemperatures may cause shorts and opens in the previously producedwiring interconnect patterns having latent defects. This problem can bemonitored using a test structure shown in FIGS. 7 a and 7 b. In thefollowing section, for the purpose of clarity, island members 409 willnot be shown in the figures that provide sectional views.

FIGS. 7 a and 7 b show a part of the test structure 402 having M2interconnect wiring 430 and island elements 432 at the top. That is, theM1 interconnect wiring is buried. As shown in cross section in FIG. 7 b,the M1 interconnect lines 408 are connected to the substrate 420 throughthe contact 424 as in the manner shown also in FIG. 6 b. An interlayerdielectric layer 433 is interposed between an M2 interconnect layer 430and the M1 interconnect layer 408. A via 434 connects the proximal end410 of the interconnect 430 to the M1 interconnect through thedielectric 433.

However, in this embodiment, in order to monitor the quality of the M1interconnect layer, after the M2 process step, the M2 interconnect 430is made discontinuous by forming an opening 436 adjacent to the via 434between the interconnects 408 and 430 and separating a distal portion438 from the rest of the M2 interconnect. The opening 432 may be filledwith an isolating material such as a damascene oxide using techniqueswell known in the art. The distal portion 438, is referred to as thefirst scan element hereinafter (also later referred to as a “tap”).

At this step of the process, an electron beam probe may be scannedacross the first scan element 438 of the M2 interconnects and islandmembers 430 to detect defects, for example, opens, shorts andintra-layer shorts in the M1 wiring. Detection of intra-layer shorts andopens tests are performed by scanning the electron beam across the scanelement 438. In one situation, if M1 interconnect line 408 is notbroken, the electrons from the beam will find a path to ground from thescan element to substrate through the M1 interconnect 408. As a result,secondary electrons will be emitted from the scan element 438, thusindicating that the M1 interconnect line 408 is still good after the M2wiring process step. In another situation, if the M2 wiring step causesan open in the M1 interconnect 408, the first scan element 438 will notemit secondary electrons and thus the first scan element 438 will remaindark. If the rest of the M2 interconnect 430 is shorted to theunderlying M1 interconnect line, the electrons from the electron beamwill find a path to ground and the shorted M2 interconnect and thecorresponding scan element will glow, indicating the interlayer shortbetween the M1 and M2 wiring at that interconnect.

In configurations constructed in accordance with the principleexemplified by FIG. 7 b, it is also possible to make use of stacked vias(vias that extend upward through multiple layers). In this manner, forexample, one or more stacked vias may extend from M1 to M3 to allowtesting of the integrity of M1 after the fabrication of M3.Additionally, redundant vias (i.e., multiple vias) may be utilized inplace of any of the single vias (e.g., 234 and 424). Redundant vias tendto result in less defects than single vias, but redundant vias haveslightly more complex design requirements.

As illustrated in FIGS. 8-10, the integrity of the M1 interconnect 408can be further detected after the fabrication of the third, fourth andmore metal interconnect layers. For example, in FIG. 8, an M3interconnect 440 having a first scan element 442 connected to the firstscan element 438 of the M2 interconnect 430 can be formed on aninterlayer dielectric 444. In other words, a stacked plug (first scanelements 442 and 438) is formed to monitor opens within a buried metallayer M1. As shown in FIG. 9, by forming a number of vias 446 betweenthe M3 and M2 interconnects, a second scan element 448 can be formed tomonitor a possible short between the M1 and M2 interconnects. During theelectron beam inspection of the scanned swath, a proximal end of the M3interconnect may be scanned. Similarly, as shown in FIG. 10 afterforming first and second scan elements 452 and 454 of a M4 interconnect456, opens in the M1 layer (through stacked plugs 452), as well asshorts between the M1 and M2 interconnects, can still be inspectedthrough scan element 454).

As illustrated in FIGS. 11 to 13, the same aspects can be applied to thetesting of M2 interconnects. As shown in FIG. 10, the M2 interconnect isgrounded through a disconnected portion 450 of the M1 interconnect. Asshown in FIG. 12, the integrity of the M2 layer can be still testedusing a first scan element 452 of M3 interconnect 440. As shown in FIG.13, possible shorts between the M3 and M2 interconnects, after formingthe M4 interconnect, can be detected using a second scan element 454 ofthe M4 interconnect.

The above described stacked test structures may be utilized to inspectburied structures. A single test structure may include mechanisms forinspecting two or more buried structures each at different levels. Forexample, a first tap may be connected to an M1 conductive line that isgrounded at its distal end, while a second tap is coupled to an M2conductive line that is grounded at its distal end. Alternatively, therecan be dedicated areas for inspecting particular buried layers (e.g., asdescribed with reference to FIG. 4 d). For example, a first area mayinclude test structures having taps to a first buried layer, and asecond area may include test structures having taps to a second buriedlayer.

Additionally, the above described taps may also be each coupled to aprobable pad. Thus, parametric information regarding a buried layer mayalso be obtained. That is, standard wafer probing techniques may beutilized on the finished, or partially finished, wafer to inspect buriedstructures (e.g., conductive interconnect lines). For example, leakagecurrent may be measured in a buried interconnect structure having ashort. Additionally, mixed signal tests may also be performed on theburied structure via the probable pad. In this embodiment, parametricinformation and voltage contrast information may be obtained from aburied structure. One such probable test structure in described belowwith reference to FIG. 31.

G. Via Chains

As is described more fully below, the second test structures 404preferably comprise rows of first via chains 500 that may be formed inone specified module (see FIGS. 14 a 14 b), and second chains 501 formedin another module (see FIG. 4 c) in portion 208 of the test die 204. Thevia chains 500 and 501 extend parallel to the edges 204B and 204D, andbetween the proximal and distal edges 208A and 208B of the test die 204(e.g., of FIG. 4 c). In a specific embodiment, proximal ends 502 of thevia chains 500 and 501 are located at the proximal edge 208A of theportion 208 of the test die 204 (e.g., of FIG. 4 c), and distal ends 504of the vias 500 and 501 are located at the distal edge 208B of the dieportion 206. It is understood that, the second test structure 404exemplified in FIG. 5 may represent plan view of any of the via chainsconstructed between any of the metal layers, such as between M1 and M2,M2 and M3 or M3 and M4, etc.

FIGS. 14 a and 14 b exemplify a part of the test structure 404comprising an array of via chains 500, constructed between an M1interconnect and an M2 interconnect, having a proximal end 502 and adistal end 504. Referring to FIG. 14 b, in cross section, via chains 500may be constructed over a substrate 506 that is isolated from an M1interconnect line 508 by an oxide layer 510. A contact 512 formedthrough the oxide 510 grounds the M1 interconnect 508, and hence the viachain 500 to the substrate 506. A series of vias 514 formed through aninterlayer dielectric 516 connects M1 and M2 interconnects 508 and 518.

As is described below, the via chain test structure 500 allowsopens-only test types to detect opens in via chains. At this step of theprocess, an electron beam probe may be scanned across exposed metals ofthe M2 interconnects at the proximal end 502 to detect defects, forexample, opens, in the test structure 404. If the via chain does nothave opens, electrons from the beam will find a path to ground andsecondary electrons will be emitted from the top M2 interconnectindicating that the via chain is good. However, if the chain is broken,the M2 interconnect will remain dark and will be registered by thesystem as a potentially defective chain for further processing.

The via chains 501 shown in FIG. 14 c are configured such that bothshorts between the neighboring chains as well as opens in individual viachains can be advantageously observed. For this purpose, the via chains501 are formed as in the manner the via chains 500 are formed, as shownin FIG. 14 b, except that every other via chain is not grounded to thesubstrate. For example, in FIG. 14 c, while a first row 520 and a thirdrow 524 of via chains 501 are grounded at the distal end 504, a secondrow 522 is not grounded. Since the via chain at row 522 is not groundedto the substrate and cannot generate secondary electrons, it remainsdark as long as it is not shorted to the neighboring grounded viachains. When the via chain at row 522 is shorted to the via chain at anext row, for example 520, the neighboring chain 520 provides a path forthe electrons to the ground and secondary electron emission occurs inboth the via chain in row 522 and 520, thus indicating a short-typedefect. Moreover, grounded via chains at the first and second rows 520and 524 are used to observe opens in the via chains as described for thevia chains 500 in FIGS. 14 a-14 b.

H. Test Structures for Defects Caused by Chemical Mechanical Polishing

As is described more fully below, the third test structures 406 arelocated on the intermediate portion of the exemplary test die 204 anddistributed along the scan direction ‘A’ shown in FIG. 5. As previouslymentioned, the third test structures may, for example, comprise arraysof isolated contact test structures, CMP test structures, overlay teststructures and CMP dummy metal test structures. Each of these structuresmay be contained within a plurality of modules.

One aspect of the present invention is able to detect defects caused bythe chemical mechanical polishing (CMP) process. As is well known in theart, the CMP process is often used to planarize profiles that build upduring multilevel deposition processes. As various layers are etchedinto the patterns during the process, the surface becomes uneven. Inorder to perform subsequent photolithographic process steps, suchunevenness is planarized by the CMP process. However, the CMP processmay polish away functioning circuit parts or cause unwanted density,pitch and or rise in the horizontal aspect ratio (length: width). Thus,it is desirable to detect defects created by the CMP process.

In one embodiment, several test structures for detecting defects causedby the CMP process are provided. Several different structures fordetecting defects resulting from the CMP process are discussed below.Typically, these test structures will reside in whole or in part withinthe scanned swath of the test chip.

1. Test Structures for Detecting Pitch CMP Defects.

The test structure illustrated in FIGS. 16 a-c exemplifies a CMP pitchtest pattern 700. In this pattern 700, test elements 702, such as metallines representing M1 interconnect lines, have substantially the sameline width 704. Each metal line 702 is separated from one another by aspace 706 that is substantially equal to the line width of the testline. Therefore, the test pattern 700 has a fixed density of about 50%line area. The test pattern 700 is configured such that every othermetal line in the pattern is grounded (see FIG. 6 c). FIG. 6 billustrates one of the metal lines 702 that is grounded to a substrate704 by a contact 710 formed through an isolation layer 708. Such testpatterns can be formed in a plurality of modules and in differentcritical dimensions on the test die.

During the electron beam inspection, as the probe is scanned across themetal lines 702 of the pitch test pattern 700, if there are no defects,grounded metal lines will emit secondary electrons indicating that alllines are good. If one of the grounded lines is open, that line willremain dark, indicating a defect. If one of the ungrounded lines isshorted to one of the neighboring grounded lines, both lines will emitsecondary electrons, thereby indicating the short between them.

As mentioned above, in a preferred embodiment of the present invention,the CMP pitch test patterns will generally be located in whole or inpart within the scanned swath of the test chip. In this manner, the testpatterns can be tested for defects during the scan of the scanned swath.

The CMP pitch test structures will preferably comprise metal lines thatare less than 20 μm in length. More preferably, such test structurescomprise metal lines that are about 10 μm or less in length. Still morepreferably, such test structures comprise metal lines that are 5 μm orless in length.

A test chip may have various CMP pitch test structures, each with adifferent pitch. The width of the metal lines in the test structureswith the widest lines will typically have lines of about 2 μm to 3 μmwide. Other CMP pitch test structures on the test chip may comprisesignificantly thinner metal lines, such as lines as thin, for example,as 0.05 μm. Preferably, a majority of the CMP pitch test structures onthe test chip will generally comprise metal lines of a width less than0.5 μm. More preferably, the majority of the CMP pitch test structureson the test chip will comprise lines of a width less than 0.2 μm.

Typically, the CMP pitch test structures on a test chip will each coverthe same area (for example, 10 μm by 10 μm or 5 μm by 5 μm) such thatall of the CMP pitch test structures have lines of substantially thesame length. However, the number of lines in a CMP pitch test structurewill be inversely proportional to the width of the lines in the teststructure. For example, if the width of the lines is 1 μm, a 10 μm CMPpitch test structure will comprise five lines and if the width of thelines is 0.5 μm, a 10 μm CMP pitch test structure will comprise 10lines.

In order to save space on the test chip, multiple CMP pitch teststructures can be placed together in a row. Generally, in such aninstance, the width of the metal lines among the multiple teststructures will vary. For example, six CMP pitch test structures mightbe placed in a row in the x-direction with each of the metal lines inthe test structures extending in the y-direction. The first and sixthtest structures might have wide lines, with the second and fifth teststructures having narrower lines and the third and fourth teststructures having still narrower lines. It should be noted, however,that in this example, it would be preferred that the “wide lines” be nomore than 1.25 microns wide.

2. Test Structures for Detecting Density CMP Defects.

FIG. 17 shows a section of a CMP density test pattern 350 to monitor theeffects of the CMP process on patterns having metal lines with differingline widths and line space between the neighboring metals lines. The CMPdensity test pattern 350 as shown in FIG. 17 has four sections (351,352, 353, and 354), each section having four metal lines. In section351, the metal lines take approximately 50% of the space. In section352, the metal lines take approximately 37% of the space. In section353, the metal lines take approximately 25% of the space. And, insection 354, the metal lines take approximately 12% of the space. It ispreferred that the lines of each section be of substantially equallength and width, as is illustrated in FIG. 17.

Similar to the test pattern discussed above with respect to CMP pitchtesting, the density test pattern illustrated in FIG. 17, is configuredsuch that every other metal line is connected to ground. As such, thedensity test pattern can be tested using voltage contrasting in the samemanner as is described in connection with the CMP pitch testing pattern.

The dimensions of each section in the example illustrated by FIG. 17 is5 μm by 5 μm. As such, the width of each of the metal lines in section351, for example, is 0.625 μm, as is the width of each of the spacesbetween the metal lines. The length of each metal line in the pattern is5 μm.

In the example provided by FIG. 17, the width in the lines in eachsection is substantially equal in both length and width. Further, thespaces in each section are also substantially equal in both length andwidth. Further, each section takes up the same space (real estate) onthe test chip. In addition, in the pattern, the width of a line togetherwith the width of the space to its right equals a constant—in thisexample 1.25 μm.

It should be noted that the pattern shown in FIG. 17 could haveadditional lines extending both to the left and the right. For example,to the left of section 351, there might be one or more sections whereinthe four metal lines are wider than the spaces that separate them. Tothe left, there might be one or more sections wherein the four metallines are narrower than the metal lines in section 354.

It should also be noted that the metal lines might be significantlylonger than 5 μm and still be within the scope of the invention.Preferably, however, each metal line will be no longer than 10 μm. Morepreferably, each metal line will be approximately 5 μm or less inlength. At the same time, it should be noted that, in a particularsection, the metal lines can be of any width that fits within the realestate allotted to a metal line and its adjacent space. For example, ametal line might take up 100% of the allotted real estate. Or it cantake, for example, closer to 4%, or even less.

Furthermore, the number of metal lines per section can vary. In thepreferred embodiment, each section has an even number of metal lines,more preferably eight. However, a section can consist of an odd numberof metal lines. Furthermore, it is possible to have sections within asingle pattern that have differing numbers of metal lines.

In a specific embodiment the density test pattern will reside within thescanned swath of the test chip, either in whole or in part, so that itis scanned and voltage contrast analysis can be applied. It is preferredthat each metal line in the density test pattern reside entirely withinthe scanned swath so that it can be scanned. However, the metal lines inthe pattern might reside only partially within the scanned swath andstill be scanned such that voltage contrasting methods are applied.

FIG. 18 provides a further example of a density pitch pattern 380 forCMP defect-related testing. The pattern comprises a plurality of metallines 381 with a plurality of contacts to ground 382. The pattern shownin FIG. 18 illustrates three different density patterns. In theembodiment shown here, the metal lines in the test pattern are 5 micronsor less in length and the widest line (Density #3) is 0.2 microns orless in width. The metal lines in the plurality of metal lines 381 arealternately electrically isolated.

3. Test Structures for Testing Horizontal Aspect Ratio.

FIG. 20 shows a section of a CMP aspect ratio test pattern 740comprising metal lines 742 having different width and length toaccommodate reasonable utilization of the real estate on the test die.As can be seen in FIG. 20, the lines in pattern 740 rise in thehorizontal aspect ratio from left to right. Also, the length increasesfrom left to right, while the width increases from right to left.Similar to the previously described CMP test structures, in thisconfiguration every other metal line is grounded to the substrate andtest structure 740 is repeated in various modules with differentcritical dimensions. As can also be seen in FIG. 20, in one embodiment,the lines are paired into lines with the same horizontal aspect ratio(and preferably the same dimensions). In each such pair (e.g., pairs 743and 744), one of the lines is electrically isolated (floating) and theother is not electrically isolated (preferably connected to ground).

It should be noted that the pattern 740 can be repeated on the test diewith increasing or decreasing space between lines. This is true of anyhorizontal aspect ratio test pattern. Such repetition (repeated testpatterns with increasing or decreasing space between lines) can beutilized to develop a row of test patterns on the substrate.

FIG. 19 illustrates another horizontal aspect ratio test pattern 720.Here the horizontal aspect ratio increases from left to right, but thelength of the lines remains the same, preferably no more than 10 micronsand even more preferably no more than 5 microns. This pattern can berepeated on the test die similar to the test pattern of FIG. 20. Again,the lines of test pattern 720 are paired into lines of equal dimension,with the lines in the pair alternately electrically isolated. Testpatterns 720 and 740 are inspected the same way as the CMP pitch teststructure 700.

FIG. 21 illustrates an additional horizontal aspect ratio test pattern750. The pattern comprises metal lines in increasing horizontal aspectratio from left to right. Every other line has a contact to ground 752.On the bottom of each line is a scan structure, which is of uniform sizethroughout the pattern 750. In one embodiment of the present invention,the metal lines would be scanned along the scan structures (in order toinsure better uniformity). However, such scan structures are notrequired to practice the present invention. It should be noted, however,that such scan structures can also be utilized in the other CMP patterns(pitch and density) described above.

I. Contact Arrays

FIG. 15 a shows an exemplary array 600 of isolated contact teststructures 602 that is located on the intermediate portion 210. Suchstructures enable monitoring of defective contacts in integratedcircuits.

Rows of the array 600 preferably extend parallel to the proximal edges206A and 208A of the first and second portions 206 and 208 of the testdie 204 (See FIG. 4 c). FIG. 15 b exemplifies the multilayerconstruction of the isolated contact test structure 602 formed on asubstrate 604 using conventional semiconductor processing techniques. Anisolation layer 606, such as an oxide layer, isolates a M1 layer portion608 (M1 pad), from the substrate 604. A contact 610 formed through theisolation layer 606 connects the M1 layer to the substrate 604.Preferably, an interlayer dielectric 612 isolates the M1 layer portionfrom a M2 layer portion 614 (M2 pad). M2 layer portion is connected tothe M2 layer portion by vias 616 formed through the interlayerdielectric 612, thereby establishing a ground connection for the M2 pad614. In the illustrated embodiment, M1 and M2 layer portions 608 and 614are connected with four vias, which will be referred to as redundantvias. In this embodiment redundant vias 616 help to monitor defectivecontacts without concerning about failing vias, thus limiting thefailure mechanism to the contact 610. This configuration shown in FIGS.15 a and 15 b assures that the random defects, in any of the vias 616,do not prevent contact to the substrate 604. As long as one of the vias616 is not defective, ground connection between the M2 pad and thesubstrate will be established.

During the electron beam inspection, as the probe is scanned along therows of exposed metal of M2 pads 614, if the contact 610 is not open theelectrons from the beam will find path to the ground and secondaryelectrons will be emitted from the pad 616 indicating that the contactis good. If the contact is open, the pad 616 will remain dark.

J. Test Structures to Monitor Misalignment

As is well known in the semiconductor industry, each step of ICmanufacturing requires precise alignment control in order to preventunwanted contacts between devices or changes in device dimensions. FIGS.22 a through 22 c show six types of overlay test structures 800 athrough 800 f used to observe misalignment problems occurring during themanufacturing process, such as those that occur during the patterning ofmetal layers. Test structures 800 a and 800 d are “M1 to contact” typestructures and have strips of M1 metal (804 a and 804 d, respectively)extending over a contact (806 a and 806 d). Test structures 800 b and800 e are “via to M1” type structures and have a M2 via (806 b and 806e, respectively) over a M1 metal strip (804 b and 804 e). Teststructures 800 c and 800 f are “M2 to via” type structures and havestrips of M2 metal (802 c and 802 f, respectively) extending over a via(806 c and 806 f). Each type includes a test structure for measuringmisalignment in the x direction (e.g., FIG. 22 a), as well as the ydirection (e.g., FIG. 22 b). Of course, any other misalignment directionmay be used.

As exemplified in FIGS. 23 a and 23 b, each test structure may form partof an array that is utilized to determine whether a misalignment hasoccurred and a measurement of such misalignment. An array of “M1 tocontact” type structures are shown in both FIGS. 23 a and 23 b. Thestructures are fabricated to be almost identical to each other, exceptfor positioning of a contact (e.g., 811 or 813). As shown in FIG. 23 a,the middle test structure 810 e has a contact 811 e that is perfectlycentered when there is no misalignment. The structures to the right ofthe middle structure have incrementally misaligned contacts in thenegative y direction. The rightmost contact 811 i is misaligned so thatit no longer touches the M1 layer. In contrast, the structures to theleft of the center are incrementally misaligned in the positive ydirection the leftmost contact 811 a is misaligned so that it no longertouches the M1 layer. Similarly, the test structures of FIG. 23 b areincrementally misaligned in the positive and negative x directions.

The contacts 811 and 813 are tied to the substrate and grounded. Thus,if the test structures are aligned, particular test structures will betouching the contact and others will not. As a result, during voltagecontrast testing, the contacting test structures should appear bright,while the non-contacting structures should appear dark. In theillustrated example, the test structures 810 a, 810 i, 812 a, and 812 i(which do not touch the contacts 811 or 813) should appear dark, and theremaining structures should appear bright.

However, when there is a misalignment, some of the test structures thatare expected to appear bright may actually appear dark, and some of thestructures that are expected to appear dark may appear bright. Therelative positions of the contacts 811 and 813 are chosen so that amisalignment may be readily measured. For example, the contacts of thearray are designed to be offset from M1 by increments of 0.0005 μm.Thus, the misalignment amount may be determined by how many teststructures appear dark. For example, if only the two rightmost teststructures 810 g through 810 i (and the leftmost structure) appear dark,the misalignment is 0.0005 μm or less in the negative y direction.

When using die to die comparison mode for defect analysis, the overlaystructures must have a set of corresponding structures of which none areconnected to ground. These ungrounded structures serve as references andare compared to the grounded structures to ensure that all connectedoverlay structures are detected. If the reference structures are notprovided the results from the overlay structures will not be consistent.

K. CMP Dummy Metal Filings

Another test structure that may be formed within the intermediatesection 210 of the test die 204 is the test structures using CMP dummymetal filings as a test pad to test for vias. As is commonly known inthe art, CMP dummy fillings are auxiliary metal structures CMP processintegrity distributed on a wafer to facilitate an even polishing of asurface of the wafer. They prevent rapid erosion of relatively softmaterials located on the surface when such materials are neighboringharder materials. In one embodiment, as shown in FIGS. 24 a-24 b, ametal filler may be converted to a multilevel test structure to monitorthe integrity of the CMP process with the addition of vias and/orcontacts.

In one exemplary test structure 902, a contact 904 is formed between asubstrate 906 and a first metal layer portion 908 (first metal pad), andthrough an isolation layer 910. The first metal pad 902 may be connectedto the metal filler 900 by at least one via 912 formed through ainterlayer dielectric 914. This structure 902 may have more metal layersabove the metal under test (MUT) and more redundant vias between thesemetal layers. During the electron beam inspection, the probe is scannedalong the rows of metal fillers 900. If a defect free path to substrateis established, metal filler 900 emits secondary electrons, indicatingthat the via and the contact are not open. If any one of them are open,the metal filler remains dark.

After the test chip is initially designed, it may be determined whetherempty space requires dummy fillings to prevent defects caused by CMPpolishing. Under current technology, this is often determined usingsoftware tools available in the marketplace. This determination will bebased on the size and configuration of the empty space. Once thisdetermination is made, according to the present invention, the dummyfillings can be fabricated as contacts that can be tested similar to thecontacts of the contact arrays described above. In this manner, theempty space is utilized for testing purposes on the test chip. Inaddition, this same method can be utilized for empty space of VLSIproducts.

FIGS. 26 a-26 c illustrate the process for utilizing dummy shapes forpurposes of testing for defects. FIG. 26 a illustrates a product chip900 with empty space 901. FIG. 26 b illustrates a pattern of typicaldummy shapes 902 used to fill up empty space on a product chip. FIG. 26c shows that contacts 904 have been added to some of the dummy shapes903 to permit voltage contrast testing (the other dummy shapes areallowed to float). Such usable dummy shapes can be included in bothdedicated test chips and product chips (for in-line voltage contrasttesting).

L. Location and Characterization of Defects

As discussed above, once a defect is detected via the voltage contrasttechnique, it may be important to determine the location of the defectand to characterize it. Where the scan of the scanned swath has been inthe x-direction, inspection of the test structure at issue may berequired in the y-direction. For example, in connection with theexemplary test chip described above, a conductive line may provedefective based on the analysis of the scan of the primary scan area. Asdiscussed above, these defect(s) may reside outside the scanned swath(i.e., primary scan area). In other words, the primary scan area isscanned first to determine whether the test structure contains one ormore defect(s). Testing may then be performed in the y-direction tolocate and characterize the defect(s).

Preferably, defects are located after completion of the scan in thex-direction. That is, all of the stubs of the conductive lines arescanned to determine if any conductive line is defective. The locationof each defective stub and corresponding conductive line is thenrecorded. After all of the stubs are scanned, each conductive line maythen be scanned in the y-direction in an efficient pattern. For example,if the stubs were scanned left to right in the x-direction, eachconductive line is then scanned in the y-direction, starting with theleftmost line. In sum, scanning the test structure along the entirex-direction (prior to locating the individual defects) allows a quickassessment of the number of defects within the test structure. Ifrequired, the individual defects may then be located and characterizedas described below.

Of course, any other suitable scan pattern may be implemented. Forexample, when a conductive line is found defective during an x-directionscan, the x-direction scan is halted and the defect is immediatelylocated along the y-direction conductive line. The x-direction scan maythen resume after the defect is located. However, this scan pattern isnot as efficient as a scan pattern that allows the x-direction scan tocomplete prior to locating the defects.

Accordingly, once the defective structures on the die are relocated andidentified, the exact location and the cause of the defects may need tobe identified. In a specific embodiment, this can be carried out usingan analysis tool. FIG. 25 shows a fundamental construction of ananalysis tool 950 for analyzing a failed test die containing teststructures. The analysis system may preferably comprise a dual beamanalysis tool comprising both a Focused Ion Beam (FIB) unit 951 and anelectron beam (SEM) unit 952. The analysis tool 950 is coupled to thesystem 10 of FIG. 1 and is a integral part of the system.

The system of 10 provides the recorded layout data for the defectivestructures of the test circuit and the type of the defect such as opensor shorts type of defects. The analysis tool 950 that is used inspecific embodiments of the present invention is able to scan the defectcontaining locations with either a focused ion beam (FIB) or an electronbeam to provide two different types of scanning action. The focused ionbeam can be used to provide localized cutting or removal of the materialfrom a defect containing location so as to expose underlying materialslocally, thereby avoiding the need for removal of an entire layer orlayers in certain instances. The electron beam can be used for chemicalanalysis using Energy Dispersive X-ray (EDX) analysis and imaging of thearea under analysis.

As shown in FIG. 25, the exemplary FIB unit generally contains a liquidmetal ion source 954 using gallium (Ga) to generate a Ga ion beam 956. Alens system 958 focuses the ion beam to a spot size on the test die ofthe wafer which is placed on a stage 959. A set of scan coils 960 isplaced in the vicinity of the lens system 958. When energized, the scancoils 960 cause the ion beam 956 to scan over the test die. The outputof ion source, lens system focusing, and scan coil actions arecontrolled by an ion beam control unit 962. During the material removal,the control unit 962 controls scanning of a target surface by focusedbeam with respect to scanning area, frequency, and time of scanning.

The focused ion beam, which is focused and scanned as mentioned above,irradiates a selected portion of defected structure. The accelerationvoltage of ion beam 956 can range from 10 to 30 kev. Current of thefocused ion beam 52 can be set between 10 pA and 1000 pA.

The scanning electron beam unit 952 irradiates an electron beam 964 onthe die in the vicinity of beam 52. The acceleration voltage, currentlevel and beam diameter of electron beam 964 are controlled by anelectron beam control unit 966. The electron beam 964 is used forimaging, e.g., secondary and/or back scattering electron imaging, aswell as EDX chemical analysis. The electron beam 964 may also be used toirradiate the area of interest on the die when that area is also beingirradiated with the focused ion beam.

As mentioned before, the wafer under inspection is mounted on a stage959, and further the stage is associated with a stage control unit 968for displacing wafer stage 959 in x, y and z directions. A detectionunit 970 may be placed at an appropriate location for detecting varioussignals generated at the surface of the wafer in response to irradiationby focused ion beam or electron beam. Although simplified in FIG. 25, itis understood that the detection unit 970 represents various detectorssuch as secondary electron, back scattering electron, x-ray or massspectroscope or the like. A signal from the detection unit 970 isamplified and inputted into a FIB/SEM computer. The ion beam controlunit 962, stage control unit 968 and the electron beam control unit 966are also connected to the FIB/SEM computer system which is furtherconnected to the system 10.

Once the defect site is relocated and marked, a chemical analysis knownas EDX (electron dispersion X-ray) is performed at the location ofinterest. The EDX analysis is performed using electron beam unit 952 byfocusing electron beam on the defective structure. Interaction of thebeam electrons with the material atoms generates an X-ray spectrum whichreveals the chemistry of the location under focus. When the X-rayspectrum of from the material is determined the most of the elementspresent at that location can be qualitatively identified, which mayprovide a determination of the cause of the failure. For example, ifthere is no tungsten at that location, the EDX spectrum should not showtungsten at that location. At his stage, if the cause of the defect isdetermined and identified, the process is completed and a new test diecan be tested. Assuming that the initial inspection has not revealed thecause of the failure, FIB unit can be used to either strip back a layeror produce a cross-section using the focused ion beam to reveal crosssectional structure of the area of interest.

After the FIB stripping has been performed, the locations are theninspected once again using the scanning electron beam imaging and/oranother EDX analysis and it is once again determined whether the causeof the failure can be identified. This process is repeated until thecause of the failure is identified. It will be appreciated that thisprocess allows rapid corrective action to be taken in real-time in amanufacturing environment because once the cause of failure in the teststructures is found, the manufacturing process of the wafers may bemodified in order to improve the yield of the wafers.

In connection with the location of defects that have been detected, anadditional technique can be utilized to locate a defect. The techniquecomprises pre-scanning the wafer under test with a dose that is designedonly to partially charge the structures. Shorter lines will have lesscapacitance loading and will be charged more completely to theequilibrium potential. Thus, the potential of floating lines will varyapproximately linearly with the length of the line. Longer lines willtake longer to charge. The lines can then be scanned.

As such, defects on the line can be recognized, for example, by usingthe following technique:

-   -   1. If the line is at ground potential, the signal level from the        end of the line will be mid level (say 50% of full scale);    -   2. if the line is full length and floating, then the signal        level will be about 75% of full scale;    -   3. if the line is partial length and floating, then the signal        level will approach 100% as the line length gets progressively        shorter.

By calibrating the signal level versus position, one can infer theposition of the end of the line and use this information to quickly movethe wafer so the defect is in the field of view to image the defect forclassification. Alternatively, each defective line may be scanned in anarray mode to locate the defect. In general terms, a first field of viewthat is taken from a first portion of the line is compared with a secondfield of view taken from an adjacent second portion of the line. In oneembodiment, the first field of view is subtracted from the second fieldof view. The subtraction result is directly related to whether thedefect is located within the first or second field of view. Thus, thedefect location may be determined based on such subtraction results.

M. Use of Voltage Contrast Testing in the Production of Product Chips

The techniques and various test structures for voltage contrast testingare not only useful in the context of test chips, but can also be usedin the context of product chips. FIG. 27 illustrates such a product chip1000. The product chip 1000 has a product circuitry portion 1001 (whichincludes the circuitry needed to carry on the functions of the productchip) and a scanned swath 1002 at one edge of the chip. It is preferredthat such a scanned swath be located at the edge of a product chip, butsuch location is not necessary in order to practice the presentinvention.

The scanned swath on a product chip can comprise any combination of teststructures of the type described above. In fact, the exact combinationof test structures may well depend on the particular circuitry of theproduct chip. Also, the exact combination may depend on particularconcerns at the manufacturing plant. In any event, the scanned swath canbe tested at a single point or multiple points during the manufacturingprocess. Moreover, during the manufacturing process, new test structurescan be added to the scanned swath at any point of the manufacturingprocess and then tested.

FIG. 28 provides a cross-sectional view of vertical taps 1004 (orstacked plugs) for testing purposes. The vertical taps shown here arecontained in the scanned swath of the exemplary product chip 1000. Shownis the substrate 1003 of the product chip. As shown, the vertical tapsor stacked plugs 1004 may be used monitor buried layer M1. Such verticaltaps are but one type of test device that can (if desired) be fabricatedin the scanned swath.

FIG. 29 shows test structures that can be included in the scanned swath1002. Included is a contacts array 1006 and conductive lines alternatelyconnected to ground 1005 and left floating 1007. All of these structurescan be tested using conventional voltage contrasting techniques, as wellas the novel techniques described herein. In a specific embodiment, ascanning device with a continuously moving stage, such as the typedescribed above, will be used to scan the test structures shown in FIG.29. The preferred direction of continuous motion is illustrated by thearrow 1008, although the continuous motion can be in another direction.Here, however, in order to take only a narrow strip of the product chipfor testing purposes, the conductive lines are preferably scanned withthe continuous motion being substantially parallel to the conductivelines rather than substantially perpendicular to the conductive lines(as is preferably applied in the context of a dedicated test chip wherespace is less of a concern).

Once the scanned swath of a product chip has been inspected usingvoltage contrast techniques at an inspection station, the total defectsnumber of defects can be counted. When a sample stage is continuouslymoved in a first direction through the primary scan area to detectshorts, for example, the number of open shorts can be quicklyquantified. An algorithm can then be applied to infer the expectedimpact on yield for the product die and this may be used to produce adefect control limit for each defect type at that layer. For example, ifthere are “n1” opens in an opens test structure that has a critical area“A1” to open shorts per die, then the expected defect level in theproduct die is given by (n1)*(A1/A2), where A2 is the critical area ofthe product die for open shorts at the given process layer. The criticalarea for a given process layer is defined as the total area of thepattern that would result in a failed device for a defect of a giventype at the critical dimension.

Once this control is established, if the defect level measured onsubsequent lots via scanning of the scanned swath of subsequent productchips exceeds the control limit, the manufacturer will know that itsmanufacturing process is trending out of control. Further, themanufacturer will have direct feedback from the scanning of the scannedswath as to which defect mechanism is causing the problem. Since thetest structures of the scanned swath provide directly the classificationof the defect, by the location and signature of the defect in a giventest structure, the manufacturer is able to immediately diagnose theproblem and fix it.

N. Utilization of Voltage Contrast Testing to Optimize Other TestDevices

The voltage control testing devices, configurations and techniquesdescribed above also can be utilized to optimize other inspectionsystems used during the manufacturing process. Such inspection systemsinclude, for example, KLA-Tencor products AIT II (Patterned WaferInspection System) and the KLA-2138 (Wafer Inspection System withUltra-Broadband Technology). Such systems detect many defects insemiconductor chips. However, some of these detected defects may not besignificant in that they do not affect the performance or operation ofchips. Voltage contrast testing, on the other hand, by its nature,identifies only significant defects (so-called “killer defects”).Therefore, the voltage contrasting techniques and structures describedabove can be used to optimize other testing systems so that such systemsmaximize the detection of “killer defects” and/or minimize thedetections of “nuisance defects.”

Such optimization will include doing the following operations:

-   -   1. A test chip or test portion of a product chip will be        inspected by an SEM inspection device in order to detect        significant defects;    -   2. test structures with defects will then be further scanned in        order to locate the defect and classify the defect according to        its class (e.g., “open,” “short” or “via open”) to produce a        wafer map of significant defects by class;    -   3. the same test chip or test portion of a product chip is then        inspected by the inspection tool to be optimized;    -   4. preferably, the step 3 is repeated several times with the        inspection tool set at differing configurations;    -   5. defect map(s) are generated based on the inspections using        the inspection tool to be optimized;    -   6. the various defect maps are each overlaid with the wafer map        generated by the voltage contrast testing and analyzed to        determine which configuration of the inspection tool maximizes        capture of significant defects while minimizing capture of        insignificant defects; and    -   7. the inspection tool is set to the optimal configuration.        The inspection tool may generate its defect map from a different        layer than the layer on which the voltage contrast defect maps        are generated. The above described procedure may also be used to        periodically calibrate or spot check the inspection tool.

The process can also be automated using standard automatic nuisancefiltering techniques and assign the “killer” defects as real and theother defects as nuisance and allow the automatic SegmentedAutothreshold and Real Time Classification algorithms to configure thetool to maximize the capture of the “killer” defects and minimize thecapture of the nuisance defects. See, e.g., U.S. Provisional PatentApplication No. 60/167,955 filed on Nov. 29, 1999 by inventors Bakker etal., entitled “POWER ASSISTED AUTOMATIC SUPERVISED CLASSIFIER CREATIONTOOL FOR SEMICONDUCTOR DEVICES,” which is incorporated herein byreference in its entirety.

One example of a system that can be used to automate this process isKLA-Tencor's product Klarity. Klarity is KLA-Tencor's automated defectdata analysis solution. It allows semiconductor fabrication facilitiesto automate analysis of defect data generated by inspection,classification and review tools. This yield analysis module allows usersto automate complex engineering methodologies using simple flowcharts ineffect, transferring expert engineering knowledge and defect analysisroutines to fab operators. Such automation enables the user to quicklyanalyze huge volumes of defect data and helps reduce the defect data setto represent only the most critical yield deterrents.

The voltage contrasting techniques, structures and devices describedabove can also be utilized in conjunction with defect classificationmethods. For example, the voltage contrasting techniques, etc. can beused in conjunction with the methods and apparatus described in the U.S.Provisional application referenced above and incorporated herein. In oneembodiment, electrical (or voltage contrast) data and optical images aresorted and grouped by defect type. The electrically obtained defectinformation may then be used to characterize the optical images (e.g.,sort into killer and non-killer defects) during the setup phase of anoptical inspection. Furthermore, wafer maps generated as a result of theinventive voltage contrasting techniques described above can be studiedand analyzed to find systematic defects. For example, if the wafer mapsof several test chips show that defects are frequently located in aparticular area of tested chips, it may indicate a systematic problem inthat particular location.

The wafer defect maps that are generated from voltage contrastmeasurements may also be used in other inspection or review tools. Forexample, the defect map is generated on a first inspection tool, and thedefect map is then used to locate the defects on a second inspection orreview tool. For example, a focused ion beam tool may be used to uncoverand view the defect, as described above. In sum, the defect map may beused to determine the defect's location after the sample is removed fromthe voltage contrast tool.

Further, certain patterns may emerge in the wafer maps that identify aparticular defective (or out of specification) manufacturing process.That is, the particular physical arrangements of the defects on thesample may indicate the type of defect. An example of a defect thatforms an edge type pattern is shown in FIG. 34. A particular type offabrication process will typically have a particular defect footprint.For example, defects that are arranged in patterns that radiate from thecenter of the chip may indicate a problem with equipment that spins aparticular layer onto the wafer. The defect shown in FIG. 34 mayoriginate from an etch process in which the etch tool has a gas entryport on one side of the wafer and a gas exhaust port on the oppositeside of the wafer. Thus, a particular arrangement of defects can then becompared and matched to a particular equipment footprint andcorresponding process step. In addition, the defect data maps fromseveral wafers across several lots can be stored in a database, andsignature analysis applied across the large data set. This would allowone to detect low level signatures in the data that are indicative ofsystematic process yield problems. A processor may be utilized toefficiently compare the defect data maps to signature patterns. Ofcourse, a user may also manually compare the maps to signatures. By wayof another example, the voltage contrast data may be utilized inconjunction with optical data to determine which manufacturing processto inspect so that the capture of killer defects is maximized.

Electrical defect data from a voltage contrast inspection may also beassociated with other types of data taken during various steps in thefabrication process. For example, electrical defect data and acorresponding optical image may be generated from a test structure aftereach process step. Each electrical defect is then characterized andassociated with one or more optical images. Process information (e.g.,identity of process step and the process machine's operating parameters)may also be associated with the each associated pair of electricaldefect and optical image. The evolution of a defect may then be tracedthrough the defect's corresponding optical images. For example, a defectthat appears after a particular process may result from such process.The process may then be adjusted (e.g., the operating conditions areadjusted). Additionally, once a particular defect type is associatedwith a particular optical image, any subsequent optical images that arecharacterized as being the same as the previous optical image can alsobe identified as having the same defect type as the previous opticalimage.

O. Alternative Test Structures

The above described test structures are merely illustrative and are notmeant to limit the scope of the invention. For example, any suitabletest structure may be utilized that facilitates efficient inspectionmechanisms. For example, a test structure may simply be a plurality ofstraight and uniform-width lines. This configuration is in contrast tothe alternating island and conductive line test structure illustrated inFIGS. 5 and 6 a. FIG. 30 illustrates such a test structure 1100. Thistest structure 1100 includes alternating floating conductive lines 1104and grounded conductive lines 1102. In this embodiment, the lines aresubstantially straight. One end (1105) of the conductive lines 1102 istied to ground, while the other end of both conductive lines 1102 and1104 project into the scan area 1101. The lines projecting into the scanarea may have varying lengths to distinguish between the two sets ofconductive lines (i.e., 1104 and 1102). Of course, the two sets of lines1102 and 1104 may have the same length.

As described above, opens within the conductive lines 1102 may bedetected by performing voltage contrast on the conductive line endswithin the scan area 1101. Likewise, opens between a conductive line1104 and an adjacent conductive line 1102 may also be detected byperforming voltage contrast on the conductive line ends 1104 within scanarea 1101. Additionally, stacked plugs as described above with referenceto FIGS. 8 through 13 may also be utilized in conjunction with thestructure of FIG. 30. As described above, a stacked plug may be used tomonitor a buried conductive layer (e.g., for opens).

The test structures that are positioned partially within the scan area1101 may also be utilized to measure other characteristics, in additionto shorts and opens. For example, the test structures may be utilized tomeasure various process parameters, such as CMP parameters. FIG. 31illustrates a CMP test structure 1112 utilized to measure CMP linethickness. As shown, a routing metal layer is utilized to connect aconductive line 1106 formed during a CMP process to four probe pads (notshown). Specifically, a first routing strip 1110 a and a second routingstrip 1110 b are utilized to force a current through an end of theconductive strip 1106 and out through the opposite end of the conductivestrip. That is, a current source is coupled between two probe padscoupled via the first and second routing strips 1110 to each end of theconductive strip 1106. A third and fourth strip are then utilized tomeasure a voltage difference between each end of the conductive line1106. A resistance value for the conductive line 1106 may then becalculated based on the measured voltage difference and current value.The width of the conductive line 1106 may then be derived from theresistance value to determine how much erosion and/or dishing hasoccurred in the conductive line 1106 during the CMP process.

Probe pads may be coupled to any of the above described test structures,as well as the CMP test structure of FIG. 31. These probe pads may thenbe utilized to measure parametric data regarding the test structure. Theparametric data may be utilized with the voltage contrast defect data todetermine various characteristics regarding the test structure. Forexample, if it is determined that a particular conductive line of thetest structure is shorted with an adjacent grounded line, the line maybe probed to determine a leakage current value. Additionally, variousmixed signal tests may be performed via the probe pads.

FIG. 32 illustrates a serpentine type test structure 1200 that may alsobe utilized to measure line resistance. As shown, the test structure1200 includes a plurality of first scan elements 1202 and a secondplurality of second scan elements 1204. Each of the first scan elementsis formed from a stacked plug coupled to the M1 layer 1210. The M1 layer1210 forms a serpentine pattern that is coupled to ground through vias1212. Each of the second scan elements 1204 is formed from an M3conductive line coupled to an M2 conductive line. The second scanelements 1204 are floating.

When voltage contrast is performed on the first scan elements 1202, eachfirst scan element 1202 is expected to have a different brightness sinceeach first scan element has a different line length relative to thegrounded vias 1212. For example, the first scan element 1202 c isexpected to have a brightness level of 100% since this element iscoupled directly to ground. The first scan elements 1202 b and 1202 d toeither side of element 1202 c are expected to have a 50% brightnesslevel, while the first scan elements 1202 e and 1202 a are expected tohave a 25% brightness level. Since brightness level is also related toline width, the measured brightness level for each first scan element1202 may be used to calculate line width deviation and/or lineresistance.

When voltage contrast is performed on the second scan elements 1204,shorts between M2 and M1 may be monitored. That is, since the secondscan element is expected to be electrically isolated from the underlyingM1 layer 1210, the second scan element is expected to appear dark. Whenthe second scan element is bright under voltage contrast, a short hasoccurred between the second scan element 1204 and the underlying M1layer 1210.

The test structures described above are arranged to facilitate voltagecontrast measurement during a continuous scan through the primary scanarea (as described above). That is, voltage contrast readings are takenfrom a plurality of conductive lines as the electron beam movescontinuously across the ends of the conductive lines. In an alternativeembodiment, test structures may be utilized that facilitate steppermovement techniques to obtain voltage contrast data. For example,stepper type test structures may be utilized with a stepper type SEM,such as the KLA-Tencor 8100 or eV300, Schlumberger, AMAT SEMvision, orHitachi CD tool.

FIG. 33 illustrates test structures 1201 suitable for stepper-typetechniques. Since voltage contrast measurements are performed on onegroup of test structures at a time, the test structures are arrangedinto voltage contrast groups. Thus, a first voltage contrast measurementmay be conducted on a first group; then on a second group; etc. Asshown, the test structures 1201 include a primary scan area 1201 and twosecondary scan areas 1203 and 1205. The primary scan area 1201 includesa plurality of test fields (e.g., 1207 a and 1207 b) arranged in anarray across the primary scan area 1201. A plurality of conductive lineends terminate within each test field 1207. A first portion of theconductive lines extends into the secondary scan area 1203, and a secondportion extends into secondary scan area 1205.

Each test field 1207 is sized to facilitate a raster scan ofsubstantially the entire test field without moving the sample stage. Forexample, the test field is sized such that a raster scan of the fieldarea results in a relatively clear image of the line ends that terminatewithin the test field 1207. The field area size depends on theparticular requirements of the stepper type SEM. Preferably, the testfields are substantially equally spaced from each other to allow a samestepper distance from one field to the next field. The test structuresmay be arranged in any suitable pattern, such as a two dimensional arrayor a check board pattern.

The secondary scan areas may include any suitable type of teststructures. As shown, the secondary scan area 1205 is arranged fordetection of interconnect defects (e.g., opens and shorts) as describedabove. Secondary scan area 1203, on the other hand, is arranged fordetection of via defects as described above. In the illustratedembodiment, the secondary scan areas 1203 and 1205 includes alternatingconductive lines of floating and grounded lines. A first end of eachconductive line stretches into the secondary scan area, while a secondends extends into the test field 1207. Additionally, the first ends ofthe grounded lines are grounded. During a voltage contrast inspectionwithin the test field 1207, the second ends of the grounded conductivelines are expected to have a different brightness than the second endsof the floating lines. For example, the grounded line ends appear brightand the floating line ends appear dark. When there is an open defect inone of the grounded conductive lines, the defective line appears a samebrightness as an adjacent floating line (e.g., dark). When there is ashort between one of the floating conductive lines and a grounded line,the shorted floating line will appear a same brightness as the groundedline (e.g., bright). In sum, when a particular line is defective, it'sscanned second end will appear to have an unexpected brightness level.The defect position may then be found by stepping down the particulardefective conductive line.

The stepper type test structures (e.g., FIG. 33) may be inspected in anysuitable manner that facilitates determining whether there are anydefects within a test structure by scanning only a portion of the teststructure and then determining a specific location of the defect byscanning the remainder of the test structure. By way of example, thefollowing inspection procedures may be utilized:

-   -   1. pre-align wafer to facilitate location of conductive line        groups;    -   2. the conductive line ends are initially positioned under the        SEM column by moving the wafer stage; (or the SEM column is        positioned over a first group of conductive line ends)    -   3. an electron beam is scanned over the first group to obtain        voltage contrast data;    -   4. a list of defect data and its associated stub (conductive        line end) are stored; and    -   5. steps 1-4 are repeated for a second group of conductive line        ends.        The pre-alignment is performed in any suitable manner so that        the SEM is able to automatically step to each group of        conductive line ends (herein referred to as “fields”). For        example, a step size is entered to ensure that the right step is        taken to get from one field to another field. The stage is then        automatically stepped from field to field.

A list of defects for each field may be recorded. The defects may thenbe located in each group of conductive lines associated with eachdefective field based on the recorded list. For example, the stage isrepositioned so that the SEM column is stepped along a longitudinal axisof a defective conductive line. Alternatively, the defects field undertest may be located prior to moving to the next field. In thisembodiment, a list of defects and their associated stubs may also berecorded for each field. Additionally, the specific defect locations maybe recorded for each field.

A defect in a defective line may be found in any suitable manner. In oneembodiment, a portion of the defective line that is nearest and outsideof the field is positioned under the SEM column. Voltage contrast datais then taken for this portion to determine, for example, if there is atransition from bright to dark within the defective line. The brightnesstransition position correlates to the position of the defect. If thereis no transition within this portion, a next portion that is adjacent tothe previously scanned portion of the defective line is positioned underthe SEM column.

Alternatively, any suitable search algorithm may be utilized to positionportions of the defective line under the SEM column. In a binary searchexample, a mid-portion between the ends of the defective line isinitially positioned under the column. If there is no brightnesstransition, it is then determined whether the defect is in a first halfof the line nearest the field or the other half of the line. Forexample, if the line is expected to be grounded at the end that isfarthest from the field and the mid-portion of the line appears dark, itis determined that the defect is probably located in the half of theline that is farthest from the test field. The search continues on ahalf portion of the currently searched portion (e.g., a quarter portionof the line is searched next) until the defect is found. A binary defectsearch mechanism may also be implemented with any other test structuresdescribed herein (e.g., the structure of FIGS. 6 a-6 c or 33).

As described above, the defect's location may also be approximated basedon the brightness level in the scanned end. The wafer under test ispre-charged with a dose that is selected to partially charge thestructures. Shorter lines will have less capacitance loading and will becharged more completely to an equilibrium potential. Thus, the potentialof floating lines will vary approximately linearly with the length ofthe line. Longer lines will take longer to charge. For example, a lengthof a floating conductive line with an open defect may be determined fromthe amount of charging or brightness level of the scanned end.

Shorts between adjacent lines may be quickly found by scanning betweenthe lines, rather than over a single conductive line. For example, anon-raster e-beam may be used to scan, e.g., in a single line scan,between two adjacent and shorted conductive lines. Preferably, the spotsize of the e-beam has a radius that is less that the distance betweenthe two adjacent lines. As the e-beam scans between the lines, asignificant change in the intensity level of the scan area (e.g., thescan area goes from bright to dark or from dark to bright) indicates theposition of the short between the adjacent lines.

Each described test structure may also include a guard ring or one ormore conductive structures to control the electrical field of conductivelines and thereby improve detectability of defects. Otherwise, aconductive structure that is not adjacent to another conductive line(e.g., an edge line within a test structure array or a stub that islonger than adjacent stubs) may appear brighter (due to a large edgeeffect) than a conductive line that is adjacent to another conductiveline. In sum, one or more conductive portions that are not under testare used to control the electric field within one or more conductiveportions that are under test. In one embodiment, conductive “guard”structures are placed adjacent to specific conductive portions withinthe test structure that are not located next to other conductiveportions of the test structure. The guard structure may be charged to apredefined potential (e.g., grounded) or left floating. Preferably, eachguard structure has a different potential than the adjacent conductiveline from the test structure. For example, a floating guard structure ispositioned adjacent to a grounded conductive line of the test structure.

FIG. 35 is a diagrammatic representation of the test structure of FIG.32 1200 with the addition of a conductive guard ring 3502. The guardring 3502 encircles the test structure 1200 so that portions of theguard ring are adjacent to outside portions of the test structure. Inthe illustrated embodiment, the guard ring is adjacent to outsideconductive portions 3504 a and 3504 b. The guard ring 3502 also includesfinger-like portions (e.g., 3506) that are adjacent to portions of thetest structure that do not extend along the entire length of an adjacentother structure (e.g., stubs 3508 a and 3508 b). A similar guard ringstructure may also be utilized with other test structures describedherein (e.g., the structures of FIGS. 6 a-6 c and 33).

In another embodiment, a strip may be placed down the center of theprimary scan area to facilitate random mode alignment. When the featuresof a scan area are very repetitive and small, image aliasing increasesand alignment is difficult. Accordingly, one or more large and uniquefeature may be placed within the scan area in the x and y directions tobe used for alignment in the x and y direction.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and apparatus of the present invention.For example, prior to performing a scan in a primary scan area (e.g.,300 μm), a large area of the wafer (e.g., 10 mm) may be pre-charged witha flood gun to establish voltage contrast equilibrium over the teststructures of the wafer. As a result, the amount of time for inspectingthe primary scan areas of the test structures may be significantlyreduced. By way of another alternative embodiment, the test structuresof the present invention may also be utilized in any suitable criticaldimension measurement tool. That is, the test structures may be formedwith structures having critical dimensions (e.g., minimum line width andline-to-line spacing).

Any other suitable charge control mechanisms may also be utilized forinspecting the above-described test structures. Several mechanisms forcontrolling charge are described in co-pending U.S. patent applicationSer. Nos. 09/579,867, 09/502,554, and 09/394,133 and U.S. Pat. No.6,066,849, which applications and patent are herein incorporated byreference in their entirety. Additionally, any suitable charged particlebeam (e.g., electron beam) inspection system and/or methodologies may beused to inspect the above described test structures and to implement theabove described inspection methods (e.g., e-beam systems and/or methodsdescribed in U.S. patent application Ser. Nos. 09/579,867, 09/502,554,and 09/394,133 and U.S. Pat. No. 6,066,849). Although the test structureembodiments are described above as being inspected with an electron beamsystem, of course, other types of systems may be utilized. For example,a photo emission system (constant or pulsed beam) may work well forcharging selected portions of the test structures for voltage contrastanalysis. Other systems may be used in conjunction with (e.g.,off-column) or in place of an electron beam system.

The mechanisms of the present invention may be implemented for anysuitable application, besides semiconductor chip manufacturing. Forexample, other applications are data disks, gallium arsenidesemiconductor devices, and multi-chip modules. In general terms, theembodiments of the present invention may be applied to any suitabletechnology for manufacturing electronic devices or any other type ofobjects having fine patterns. Furthermore, techniques for characterizingdefects and/or creating an x and y map of defects may also be appliedacross an entire wafer to facilitate detection of systematic processproblems. In other words, defects may be mapped across the wafer. Whenan specific area of the wafer is found to have a significantly highernumber of defects than other areas, it may be determined that a processis performing improperly for such specific area. The process may then beadjusted to subsequently reduce defects in such specific area of thewafer. Additionally, the above described test structures may be formedon any suitable portion of the wafer, such as within the scribe line oron any portion of one or more dice.

Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

1. A method of inspecting contacts, comprising: scanning a firstplurality of contacts with an incident charged particle beam anddetecting a first portion of charged particles emitted from the firstcontacts in response to the incident charged particle beam; generating afirst image from the first portion of charged particles; scanning asecond plurality of contacts with an incident charged particle beam anddetecting a second portion of charged particles emitted from the secondcontacts in response to the incident charged particle beam; generating asecond image from the second portion of charged particles; comparing thefirst image to the second image to obtain a difference image; and usingthe difference image to determine whether the first contacts, the secondcontacts, or any of their underlying structures have an open defect or ashort defect.
 2. A method as recited in claim 1, wherein the firstcontacts are formed on a different die than the second contacts.
 3. Amethod as recited in claim 1, further comprising: for subsequentlyprocessed contacts, using the difference image to further determinewhether the process for fabricating the subsequently processed contactswill be out of specification; and adjusting the process when it isdetermined that the process for fabricating the subsequently processedsamples is out of specification.
 4. A method as recited in claim 1,wherein using the difference image comprises: determining a number ofdefects based on the difference image obtained from the first sample;determining an expected defect level based on the determined number ofdefects obtained from the first sample, and determining that the processis out of specification when a defect level measured on the subsequentlyprocessed samples exceeds the expected defect level.
 5. A method asrecited in claim 1, wherein the first scanned portion is from a firstdie and the second scanned portion is from a second die.
 6. A method asrecited in claim 1, wherein the first scanned and the second scannedportion are both from a same die.